In the PowerMac era, the PRO used to mean Professional.
The Pro series was meant for high-enders, who would requier the best technology available for their work and who would be willing to pay top bucks for this.
.. the new Pro series has glossy display (no anti-flare display) on the 15″ model and the new arrival in the Pro family (the 13″ MBP) has no dedicated video card.

The Pro is no more what it used to be..