I see bottles filled with water left outside (in the gardens, by the gates). Any idea what they are for? I very much doubt they are left for any thirsty passerbys

We were told they were for dogs to pee on, the usual wall coating can be badly stained and affected by dog pee, so the bottles are a better target for them.

You should have said they were to drink from.

They are there to stop mozzies from going into the house, Apparently they see a large image of themselves in the bottle of water and fly off fast.

Rosemary

I was told they deter cats https://www.mirror.co.uk/lifestyle/g...g-your-1324378

I always thought it was to stop dogs peeing as they see a reflection and move on.

I heard in Italy that it was to deter cats

Indeed.
Anyone I asked always said it was to stop dogs urinating there as if one does they all do.
Have never seen a dog myself pee on the bottles but as you know they will do so on anything given half a chance.

Around here people put bottles around the base of lamp posts to STOP dogs peeing there.

So which is it - to encourage or to stop dogs peeing .....

Probably another myth.

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They pee on the bottles so it doesn't get into or onto the lamp posts, when Iron lamp posts were first used in London, they fell down after few months, just rotted away, this was because pee is acidic and was dissolving the Iron, perhaps the reason for the bottles many years ago?

So why would you encourage dogs by putting bottles around the base of the lamp?. I don't think dogs aim too well so some is bound to go on the post itself.

I walk with our cat around the area and he actively rubs his face on water bottles, not bothered by them in the least.

I don't think you need to encourage a dog to pee on a lamppost, it seems to be genetically inbuilt. For the same reason I don't think you can discourage them either. All you can do, is assume that dogs will pee on your lamppost and try to limit the damage. Or don't put up any lampposts but then you risk treading on the other stuff that dogs do.