Center tension

In "Chess Master vs. Chess Amateur," the authors, in discussing the French Defense, stipulate that "in general, it is best to maintain tension in the center as long as possible," but they fail to tell us why. So I put it to you guys and gals, "Why?" Thanks.

Maintaing the tension means keeping the pawn structure undefined by not exchanging and doing so only at the appropriate time.
This way the opponent is kept guessing on the piece placement and over all plans.
Hope that explains

My guess is that some pieces, the knights and the bishops, are 'more valuable' in the center than they are at the edges of the board. They can cover more squares when centered.

Originally posted by Aiko
My guess is that some pieces, the knights and the bishops, are 'more valuable' in the center than they are at the edges of the board. They can cover more squares when centered.

What you have stated is true but it is not what is refered to. The second poster had it right, but I will attempt to further clarify his answer.

Center tension in the french is almost always maintained by blacks pawn on c5 and whites on d4. White doesn't want to capture dxc4 as it weakens his center. In some lines, black plays cxd4 and in others pushes c4. Tension is maintained by doing neither for as long as possible. (In my experience) White against the c4 push can usually mount a kingside attack and can play easily on the Queenside after ..cxd4 when he plays cxd4 or Nxd4. Not knowing which of these is going to happen white cannot be sure of where to best place his peices and must guess .