I’m interested in finding out when and why little traditions and things we take for granted started?

[Britpopbaby]

Like…
Having ice cream at the theatre.

Popping money in a purse before gifting the purse to someone ( I think this might be geographical)

Bringing sweets back from your holiday.

These are weird examples you give

this

@devilla how are the OP's examples weird?

I love this idea for a thread :)

In our family, we have 'pressie practise' on Xmas eve after supper, essentially a family Secret Santa of silly gifts before a sensible grown up gift each the following day. Always makes for a laugh. Even the dog gets to 'take part' ha.

When I first moved to London years ago, I lodged with an older lady who I struck up a friendship with - I moved out aged student-y 22yrs old after 3 months, bit still to this day aged 33yrs, she writes letters and sends me a mug for my birthday because I became known as "the mug hoarder" haha (I.e. I used to end up with all her tea stained mugs in my room for a week at a time... oops...) love her to bits

No they're not.
And also, why be such a Grinch?

DSis and I have a weird one. We both have dogs and sometimes go to the same places to walk them, but unplanned and arrive at different times. It’s now compulsory for the one who leaves first to lift the windscreen wipers up on the other’s car.

The ice cream at the theatre is a bit peculiar. I guess it's not noisy to eat. Or smelly.
I reckon it's a simple as an ice-cream company rep pitching/ selling it to a company that runs theatres and it took off. I always remember it was always the same one brand in all the theatres in London. I guess they worked out it was profitable. Small volume per product for a high price?

Someone will Google it I'm sure but that's my boring guess.

Popping money in the purse is to ward off money problems, ie if you give an empty purse it’ll never stay filled. It’s a wealth ritual of the same thinking as tucking a coin in the baby’s pram, basically wishing them prosperity.