For me, and I suspect many others, the big deal is that it makes env management simply disappear. Before uv I used conda+poetry for years, and there was always the need to activate the env before doing anything (I used autoenv and ended up with .env files containing "conda activate <env_name>" in every project), and various other small pains that I actually became accustomed to over the years, but then I'd always be surprised when someone said X didn't work when it was fine for me. The uv came and I felt a great relief that I didn't know I was missing, and suddenly there were also no more surprises.