Dual Supply Voltages in Op-Amp Circuit & Op-Amp parameters
Why do we need dual supply voltages in Op-Amp circuits ? 


You need dual supplies if your signal has a negative voltage portion and a virtual ground (DC offset superimposed on your signal) is not suitable.


  Which parameter of Op-amp causes to make a square signal's shifting between 0-1,1-0 slower?


The slew rate defines how fast an op-amp can change its output level.