NPCs can truly change a campaign in the best ways, especially if they grow with the campaign as a whole. In one Star wars campaign my Jedi Ana Tathis was the target of a Dark Jedi that wanted to turn her to the darkside, she defeated him time and again but he always came back because she found he was in love with her. while she never managed to turn him to the light she did redeem him enough that he sacrificed himself to save her and some rebels she was protecting

In another Grey BIshop my dark jedi assassin in the vein of Galen Marek from the force unleashed fell in love with Valeera the Slicer from the Heroes and Rogues supplement. what was meant to be a one time encounter and save her and get her off planet became a galaxy spanning adventure that took the group deep into the Corporate sector to save Valeera and get her life back. She would become a full time member of Greys crew and the two would eventually marry when the Empire fell,.

NPCs done properly as well as adding distinctive worlds and such bring so much fun

I was able to integrate the Eldar and Tau and Orks into a Star Wars run even using the Yuzzan Vong which had tyrranid style bioships and creatures as weapons. gave the pcs an epic space battle with those and an epic scene of one of the PCs wookie character ripping some orks arm off and beating him to death with it. =D

Absolutely.
NPC's really shape a campaign better than anything else. As human beings we're social creatures, so the relationships we form with others are far more important to us than places or objects.
For example, I have no memory at all of where our Rebel cell was based in our campaign in the late 90's. But I remember our commanding officer, a Noble Lady from a distant sector of space. She carried a sword rather than a blaster, and constantly seemed to be disappointed at the disorganised rabble we were.