The big difference is 1. Finding communities is not easy. Can you name every one of your interests off the top of your head? 2. Just punching in communities from reddit 1 for 1 doesn't work. 3. Content and user base. There are no discussions for my favorite podcasts here and if there are where do I find them.
It took me a decade to build out a decent sub list on Reddit and I still stumble upon new interests now and again. I don't know how to "stumble upon" decent communities on Lemmy and I'm sick of wading through cartoon horse cocks.