Personally buying players during the playoffs in progress (without participating in them), it seems to want to take advantage of the increase in costs at the beginning of the season.
So in my opinion there is a reasoning: you can try to save on purchases, but in the meantime you pay your salary for a week that you don't play.
There are almost 5 days to shop between the last economic update of the season and the first game of the following season (in which the form will not change even if taken before the training update).
In reality, in many leagues, purchases cannot be made during certain periods of time.
I would have always liked more "break" between the end of one season and the beginning of the next .. just to be able to think and fine-tune the squad without having to hurry .. but I see that most of the managers there think differently ..