Many "Old School" or "High Intensity Training" size routines have stiff leg deads for high reps. Sometimes the 20 rep squats are followed by 15- 20 rep stiff leg deads.
JMaier mentioned, the concentric or lowering for mass. In the stiff leg, this part of the lift is "automatically" emphasised. Also, the ROM is longer, maybe more muscles in your back get worked or something?
If you want to bring up your lower back without hurting yourself, you could try Deadlifting for "regular" reps, then do high reps in a more targeted/isolated move like back raises. That way you can use weight and reps that are just right for your lower back, not too heavy.