I am not a guru in the Tarrasch, but just plain common sense says that you are developing a piece for Black without it costing him a tempo. Similar to the orthodox QGD where Black waits for White to move his bishop before taking on c4 and making him move it a second time.
Same concept.
When White isn't really developed yet, the pawn on c5 will be relatively easily regained by Black. The move 4 dxc5 isn't that bad, but doesn't put serious pressure on Black. After 4 cxd5 exd5 5 Nf3 Nc6 6 dxc5!? is a really dangerous option, however, because White obtains the bishop pair and some free developing moves. Also, Black has been doing well in the main variation after 9...c4!, but that's admittedly not entirely on-topic.
Thank you so much for your answers. I think I get your points. <3
Why would somebody playing the Tarrasch be happy to see 4. dxc5 instead of the book move 4. cxd5 ?