Why is important for three phase supply voltage to be in-phase?

The phase voltages (i.e. line-to-neutral in a 4-wire system, or line-to-line in a 3-wire system) of a three-phase system are displaced from each other by 120 electrical degrees. Each phase voltage, in fact, is rarely in phase with its phase current, as the phase-angle (the angle by which a phase current lags or leads its phase voltage) is determined by the load not by the supply.

In general, most loads are resistive-inductive so the phase current usually lags the phase current.

So, to answer your question, it's not important that a phase current is in phase with its phase voltage -if that is what you are asking.