Hhy should I need this mod on client side, it just spawn structures like https://modrinth.com/datapack/create-structures which isn't needed on client-side.

i am not sure because i havnt tested it out yet but the mod also adds some blocks and items which makes it also client side. i think
not sure

i just tested it and because of the blocks and items both the server and player needs to have it installed

please mark the mod as server and client required on curseforge