Your_average_minecrafter Not much. Most of the time, breaking changes only happen in the snapshots. I just think it's nice from a user perspective to have a dedicated build for each version, so you don't have to guess which older build might be compatible.
Also, there is no real way for me as a developer to know if a pre-release / release candidate breaks Wurst until I try to port Wurst to it. So at that point, I might as well release the result. ¯\_(ツ)_/¯
Although in the 1.20.2-pre3 build, I did fix a potion bug that went unnoticed during the snapshots.