Alfred is the first app many people install on a Mac: open apps, search the web, reach the clipboard, all from one input bar.
It can find files too, but it relies on the system index. If a file is on an external drive or a NAS, whether it turns up depends on whether Spotlight has indexed it. If it doesn't, you switch tools and search again.
Locseek saves you that second search: your whole Mac, external drives, USB sticks and NAS in one list.