Going to Japan in a few weeks and I’m not sure if I should get the JR pass beforehand

So, what the title says. Many reddit posts say that I should purchase the JR pass before going there, some other say that I shouldn't. My idea is to kinda wing it, and get train tickets and stuff once I'm there, coz I haven't fully decided my itinerary. I'm going to Osaka and flying back from Tokyo, that's all. What am I gonna do in between, my plan is to decide it once I'm there. Does this make sense, or is kinda ill-advised?

Thanks!