What is edge trigerring and what's need for it?
Something that triggers on an edge only triggers for an instant. Something that triggers on a logic level will trigger continuously as long as that logic level is there, limited by the propagation delay of the circuitry.

Suppose you wanted a counter to count up once per clock cycle. If it triggered on a logic HI, that's no good because it might trigger thousands of times while the clock signal is HI before it LO again. What you really want is for it to trigger just once when it goes HI and then stop, and not trigger until it goes LO then HI again, but that's what an edge is.

You basically can't level trigger off of anything that is a clock because nothing will stay synchronized. You need to edge trigger.