I would like to have some help to understand sticky bucketing better, after reading the information here https://docs.growthbook.io/app/sticky-bucketing, I got a bit confused because in the introduction it says

(1) Bucketing based on either a primary hash attribute (i.e. user id) or a secondary attribute (i.e. anonymous id) and (2) the ability to version-control and purge your users' assigned buckets.

but when checking the implementation it is using cookies so now I am wondering why exactly does it need cookies if it is based on the id for example. How does it really work?