From the minimalist Store description:
Edit your video before sharing it! Apply filters, add music, trim, crop, rotate, flip, mirror, change speed, adjust exposure / contrast / saturation / volume. Make your video better!
The promo video below does a good job of showing what you can do (to a single clip, typically shot using the phone's camera):
The Conversations blog somewhat overplays the functionality:
Editing videos you’ve shot from your smartphone has typically required the need to get it off your device and onto a computer. Then, you’d need either a) some expensive editing software b) to use some clunky freeware. With the Video Tuner app, you can keep everything in one place, making the process much quicker, easier and, best of all, free.
Once installed, you have instant access to a set of basic, yet powerful, tools, such as trim, crop, rotate, flip, mirror, and speed change. Delve deeper and you’ll find more advanced features, such as adding music to your video and adjusting the volume to suit particular scenes. Need to adjust the exposure, contrast, and saturation levels? Yep, you can do that, too.
Thanks to the likes of Instagram, we’re all well-versed at using effects to change the look and feel of our images. The same applies with Video Tuner, just select your filter and away you go.
It's a perfectly valid thing to want to do, of course. And Video Tuner also allows the trimming of a video, so think of this as a more fully featured replacement for Nokia Video Trimmer.