Pinion angle is another reason people put in adjustable trailing arms.

When you lower your car, your diff nose no longer sits in the correct position (points down), thus moving some point of peak force or some shit... someone explain this better....

Basically when you're diff nose is sitting level, you win the prize, less wheel hop, more power down etc. So you put in adjustable upper trailing arms to adjust the pinion angle without affecting wheelbase, or upper and lower to rectify pinion angle AND adjust wheelbase if that is your desire.