Because they just spent several hundred to several thousand on an Amp and feel a need to justify it.
But seriously, sometimes Speakers need some additional current behind them. Some woofers and the XO network can create huge demand for power at low impedance. It really depends on too many variables, and you need to look hard at the electrical behavior of the Speaker in question to even know if it
might benefit from an external amp: Think larger woofers at low sensitivity... Bad phase angles and low impedance...
My gear is supposed to present a relatively benign load to drive. I do use external Amps, but never expected any sort of SQ enhancement. For me it was just about headroom; having the power on hand to handle dynamic peaks in music and movies.