The first thing that comes to mind is multi-platform support. Currently, it's very slow to build multi-platform images in things like Cloud Build because you have to emulate the Arm portion of the build. We don't have timings in Cloud Build for this, but in GitHub Actions a multi-platform image build can take over an hour when building for both Intel & Arm. With Depot, those builds can take less than 5 minutes because we build the image for both architectures in parallel on native CPU architectures.
Other things where Depot might be more streamlined than Cloud Build is that persistent caching is automatic. The dedicated build machines for each architecture have persistent SSD caches attached to them, so you don't need to save/load cache in between runs, it's just there ready to be used.