Working productively with information usually involves multiple applications. Depending on your personal workflow, not everything might be available for searching in DEVONthink. Here’s where HoudahSpot comes in: It uses Spotlight to search your Mac but offers way more features than macOS itself so that you find precisely what you’re looking for.
Of course you can find documents stored in DEVONthink too. You just need to enable Spotlight indexing for your databases in File > Database Properties. Our friends at Houdah have also written a great blog post about how to search explicitly only in DEVONthink if needed.