Most older (and also some newer) action cameras can only record in files of up to 4GB. When recording gets larger they split it into 4GB fragments. This is annoying if all you want to do is upload all of them to youtube or something. This app is made to join them back.

Naturally, this can be done in any editor but unlike them this app does not process the video. It only puts frames as they are encoded by the camera in a single file.
There are two main advantages to this — quality and speed. Because no transcoding is taking place there is absolutely no loss in quality and the process is very fast. At least an order of magnitude faster than putting fragments into a timeline in a video editor end rendering it. The speed is basically limited by your disk's write speed.

The app also lets you change the frame rate of the video. It is useful when you have a 30fps time-lapse and want to convert it to 60fps without re-compressing the video.