Congrats! And thank you for your time to this project! It's awesome!

i wasn't aware there is such an entity. What are the benefits?

@bogo I think @mkyral may give you a better answer, but I don't think there is any benefit from the membership besides being able to participate in association voting. At least I didn't expect any. I've been contributing to OSM for some time and I thought it was time to support development of OSM in CZ by becoming a member of the local OSM association.
AFAIK association was founded to have some legal entity to negotiate open data access with government bodies. It's better if they're approached by an association than just a bunch of volunteers.