I'm told that 60 years ago, a lot of players showed up having done little to stay in shape, so there was a need for that aspect, conditioning. Nowadays, nobody shows up except in top shape, no one (and they are still sore for a week or so). I think it's mostly for the minor leaguers now to get some time to show their stuff. The major leaguers play little, work on some basics to stay sharp.
I know when I was 25, I could get in pretty good shape in a week, easy. I can't do anything like that now of course. But they all say it lasts too long today, the are ex major leaguers.