The Tarrasch Defense was the brainchild of the Father of German Chess, Siegbert Tarrasch. He did not even look at 3...Nf6 or any other move. He held that 3...c5 was the only rational way to proceed.
This view was based on his belief that the isolated d-pawn that Black would be left with after 4.cxd5 exd5 5.dxc5 Bxc5 would be a huge asset. Black would have more active pieces in the middlegame and this greater activity is something White should not allow.
Instead, Tarrasch maintained, White should play 4.e3 and wait for a better moment to break the tension. Furthermore, he insisted, Black should resist the temptation to make the 'same mistake' by 4...cxd4 5.exd4 dxc4 6.Bxc4 giving White the same 'asset'.
Assessments on the pros and cons of the isolated d-pawn have a major bearing on whether you should play this line. If you agree with him that the isolated d-pawn is a strength, adopt the Tarrasch. If you believe the isolated pawn is a liability in the endgame give 3...c5 a miss. Either way knowledge of this opening will help you effectively play against the lonely d-pawn.