Why the battery need to be loaded before to root? I read in some tutoriel that for the root to be working, battery needed to be loaded around 70 at at least. 

Why is it so?
Does it endanger the phone, or it just prevent the root to be working for some reason?
It's a safety factor.  Depending on the exact method, the rooting process can take a hour or more and if you run out of juice in the middle of the process, your phone can wind up bricked.