Yes I agree I can see why they would want to check details on the old passport although one would think those details would be held electrically given they scan it when required at border checks. I presume simple reason for sending it back all be it with corner clipped off to show it is no longer valid will simply be because it's your property bought and paid for .
My wife was born to RAF parents in France she has 3 birth certificates , French , British and a NATO issued one .

How does it make it easier?
I don't care about keeping it ..... I care about not having a passport whilst waiting for the replacement.

At the moment the process is around four weeks, unless you travel for work. I think most people would be patient...

The other thing now with the UK is that you don't get any extra added to the 10 years, so unless your passport is within 3 months of expiry it's not worth renewing early.
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I've only been forced to renew early as Canada would take my passport for a work visa as it only had six months left.

Didnt it used to be that one had to visit the local British consulate to renew your passport if living outwith the UK? If so then an online application and posting passport back to renew i consider much simpler. I do get that its a concern sending it in case it gets lost then what re the renewal? As I mentioned earlier my recent renewal experience was excellent 2 weeks from submitting application online to receiving new passport a few days longer for my old passport to be returned and updated electronically throughout the process.
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Fred explained why you need a passport and I can think of several other reasons why you might need to keep your passport to prove your ID. What about an unplanned vacation or urgent trip to attend a wedding/ funeral. I cannot understand why they need you to surrender old before you receive new. All info they need is stored online. I doubt they use your old? Waiting for it to arrive and logging receipt and forwarding it to the correct staff processing your application will surely cause unnecessary delay and hassle for them

"I cannot understand why they need you to surrender old before you receive new." - probably because you cannot have two valid passports? Can you have two valid driving licences? (Just wondering, but when you get a new credit card and you use it in a shop/petrol station, does your old one stop working?) - just before someone possibly splits hairs and and says you can, I mean the replacement for the old one and the old one, not two different ones.

I agree that not having a passport could well be an issue should an emergency arise during the time that you have sent the old one back and the new one arrives. I too find it odd why they need the old passport to verify details given surely they hold all of that information. Maybe they simply want to clip the corner to physically invalidate the old passport ? I renewed mine recently because of the recent 10 year rule although i still had nearly 6 months to the expiry date. I suppose theoretically had it not been returned and clipped i could have had 2 passports simultaneously for a few months not for border checks but physically had 2 showing unexpired. For emergency purposes it would be good if somehow when applying to renew there was some sort of electronic short term set up possibly to phone computer etc confirming the old passport was sent yet allowing emergency travel. i guess the only way around that now would be a visit to a nearby consulate and obtain a temporary travel document, that would be stressful say it was needed for a family emergency and i would guess very time consuming. Its certainly an issue though for the few weeks ine doesn't physically have a passport in their possession.

A copy of your old passport that has been seen and stamped by the Guardia is an acceptable temporary solution to the problem if it arises. However, this would not be acceptable for travel purposes but OK as ID.

Yes that works for ID to be fair the odd time i have needed ID the TIE has been fine. As Missile says though there could be a family emergency requiring your attendance crop up in the uk. Probably slim chance but it could occur and sods law it would be while your passport is being renewed.

I just find it strange that there is such an inconvenient rule which serves no purpose.
An out of date / expired passport is of no use to a criminal and like many government rules it only inconveniences honest people.

Sods law, dictates I am on the toilet when I get an Amazon delivery :-)

A quick google as to why a passport has to be returned.
So they are checking the passport itself - which for a variety of reasons may differ to the electronic records.
This could also include travel information that will not be on the electronic records.

On the point of returning the old one, I notice the Canada answer adds So I guess that means Canada doesn't clip the corner and send them back cancelled.

Actually maybe you should care - because I have heard it said (can't quote specific reference) that if a passport number has been used in a escritura and then later when you sell you may need to produce the same passport even if it has expired - all because UK passports do not keep the same number. Just a thought.

I think you'll find the criminal fraternity will disagree with you on that!

The one positive thing I do have to say about the new passport is that it is widely regarded as one of the most, if not the most, secure in the world and all but impossible to forge or alter without easy detection but it will not be until 2030 that all old burgundy passports expire and become truly worthless to criminals simply because of their legacy form factor.

Still hate the new one though

I did google before I exchanged my own passport. That is just facile justifications which make no sense to me.
So they are checking the passport itself > Perhaps they did many years ago when verification was done by a human. I do not believe they do this now(?)
which for a variety of reasons may differ to the electronic records. > What reason(?), I cannot think of any ....
This could also include travel information that will not be on the electronic records. > Why is this relevant? I really do not believe passport office are checking for stamps for entry to a.n.other countries. Governments do exchange travel information and this is available "on the electronic records"

Perhaps you know more criminals than I do? I would suggest any criminal using an out of date passport would be easily spotted :-)

Really, did you read that in the Daily Fail :-)