That may not be something that immediately sounds useful, but it does have its uses. You can create a seamless audiobook, merge ripped CD tracks into one or make a mix for someone (a mix where they can't skip tracks!)
Merge MP3 can import ID3 tags from any of the tracks being merged, or you can write a new one yourself, and listen to the tracks in the program. There is no limit to the size, length or number of tracks than can be merged.
The interface is very simple, and merging files is super fast.
Merge MP3 does what it claims perfectly and efficiently. However, because of the its limitations, it won't be for everybody. Programs such as Audacity can also merge MP3s, while also allowing you to edit, cut and mix them together, although for simple joining of MP3s, this is much easier to use.
If you want to join MP3s together, Merge MP3 is a simple program that does exactly that very well, and no more.