When it comes to Tchouameni, I don't know if Real Madrid need a player like him now. He is a defensive midfielder and they already have Casemiro in that position. However they might still see it as an investment for the future of course. He is a promising player surely.
As far as I understand, it seems that Real Madrid's goal to have Tchouameni is to prepare for regeneration, as we all know that Modric is already 36 years old, and regeneration will be needed in the future. With Tchouameni still very young, this will be a very mature preparation for Real Madrid. FYI, not only Real Madrid, even PSG and Liverpool want this player to play at their club next season.