Together with Final Cut Pro, Apple wants to have Mac-only exclusive software that is aimed specifically at cool creative professionals to build up the image that trendy creatives use Macs (and you therefore have no choice but to use a Mac, otherwise you won't have the software you need).
Aperture couldn't really compete with Adobe's whole workflow (since Apple didn't have a full-fledged Photoshop competitor). But also, movies and music are "sexier" in a way.
AVID Pro Tools works on Windows so it doesn't do anything for Apple. The whole point is awesome software that works only on Macs.
The day that every single program creatives use runs as Windows as well as Macs, is the day it becomes a lot harder for a lot of people to justify buying a Mac. It's that simple.
(So it's certainly not about any profit from the software directly, and it's also not just a marketing "halo".)