Sketch's main benefit is that it's built from the ground up to focus on vector-based design and composition, with a more modern, simplified approach to its interface. It makes it feel really light-on-its-feet and productive for UI design.

Photoshop is a powerhouse but has a lot of historical bloat built up over the years particularly around high-resolution bitmap editing. I mean it can paint a car out of a scene with a single brushstroke for chrissakes. Sketch has none of that, but in turn its vector feature set is super streamlined and thoughtful, so it's basically a UI designer's dream.

It would make a little more sense to compare it to Fireworks (may it rest in peace), as that was maybe a bit more suited to UI design than Photoshop.