Tarrasch Defence question

With regards to the following move order, 1. d4 d5 2. c4 e6 3. Nc3 c5 4.Nf3 Nf6 5. cd5 ed5 6. Bf4 Nc6 7. e3 Be7 8. dc5?

My question is what reason or benefits are there for white to play 8.dc5?

Chess 101 instructs player to capture towards the center. In this move order, white is capturing away from the center.

Although I wouldn't play that variation against the Tarrasch, the reasoning behind 8.dc is simply negating the 7...Be7 move, similar to the logic behind ...dc in the Meran after Bd3 is played.  

First things first; 4....Nf6 5.Bg5! is good for White putting immediate pressure on d5. This is why Black plays Nc6 before Nf6 and it's a good trick to remember.

The idea of dc5 is that since Black already moved the bishop he'll have to move it again after dxc5 losing a tempo. This is why in the mentioned Slav or queen's gambit White will play moves like Qc2 or Rc1 before moving the light squared bishop. White is trying to avoid something like 1.Bd3 dxc4 2.Bxc4 moving the bishop twice. It's the same here. 

In these isolated pawn positions the side with the isolated pawn doesn't usually have a way to avoid this since they need to develop quickly. 

Thanks for the replies. I understand now that the reason for white playing dc5 is to save a tempo for white. ie white avoids moving the dark square bishop twice.

There are also critical ideas for pressuring d5, usually in conjunction with a3, b4 and Na4 ideas (to control c5). Black was struggling here for a while, until he found 8...Ne4 (either on move 8 or the next one, I can't remember exactly), sacrificing d5 to reach a rather drawn ending.