Well Photoshop was created by photographers, with 'darkroom' technique stuff in mind. So at the root of it, it uses photographer jargon and had the expectation of mostly being an image editor.  All the other cool stuff has been added on over the years

 

Painter was created specifically as a painting tool for real media artists to get the real media experiance. the interface (used to be) simpler and the jargon is more artist than photographer.  ie  friskit vs mask

 

Why use both?  photoshops brush dynamics come close to painters real media brushes now, but they didnt used to.   So these days it comes down to easier and more comfortable. Some people like one, and some the other. While some find a certain tool works more to their liking here than there.

Lyrra