It's a tactical question, consider a simple example:
. . . . .
. X B # .
. . # # R
. . # # Y
Where X is an enemy adjacent to you, B is you, R is your Rogue Friend, and Y is another enemy. # represents difficult terrain. "." represents normal terrain. This assumes Y is at least as strong if not stronger then X. If Y is instead a minion, and X is a solo not much point obviously.
If you mark X, then X will attack you (the alternative being to move by you to get to the Rogue at a penalty, which in this case would almost certainty take either two actions or an OA). Y will assuredly attack the Rogue.
If you mark Y, then X will still attack you (since the alternative is wasting his turn and giving you a free attack), and Y has to choose between trying to hurt the Rogue at a -10% hit chance, and shifting/moving into the difficult terrain to attack the Battlemind instead. Now this is a more difficult choice for Y.
It should be clear that Case 2 is tactically better then Case 1. Many similar cases can come up in game play, this is just an arbitrary example.