Unlike many competitors at the time, PSP7 allowed users to work with both bitmap and vector objects seamlessly in a single program.
Included as a free addition with the 10th Anniversary Edition, was designed specifically to help web artists create high-quality animated GIFs.
One of the most-cited reasons for its continued use is its "blazing fast" performance. It loads almost instantly on modern hardware and handles complex layers without the lag often found in modern suites. Animation Shop 3.04: The Perfect Companion
My darkest secret: I still use Paint Shop Pro 7.04 from year 2001.