Safari Technology Preview takes a snapshot of the WebKit nightlies, about once a fortnight, and packages into a working web browser. Safari Technology Preview is not going to be as stable as a formal public release but the idea is that it is stable enough, such that web developers could use it as their primary browser.
By comparison, WebKit nightlies are often very buggy and using that as your daily browser is just unfeasible due to the high chance of it crashing. Safari Technology Preview also packages in iCloud support and automatic updates through the Mac App Store (even though you initially download the app from a website).
It’s a convenient way to stay closer to the bleeding edge of web technologies. If this sounds familiar, it’s almost identical to the strategy Google uses for Chrome, with Chrome Canary builds. Safari Technology Preview (with its annoyingly wordy name) also comes with a slick purple sundial its dock icon. So there’s that.