They are "intelligent" timers, so they offer a lot more features than a simple timed on/off switch. The three features I see mentioned most are F-Stop timing, Dry-down compensation, and automatic test strip timing. They have a good reptuation for vendor support and quality products.
Stunning piece of kit. More genuinely useful features than you can shake a test strip at. The best money I've spent on darkroom gear. Unreservedly recommended... ...and no, I'm not on commission!