In addition to what everyone else says (Unless I’ve missed it somewhere in one of the posts), it is also very handy for mounting stuff like lights, cycle computer, gps, bell, brake handle and water bottle. Any of which will help with distance riding.
Plus, if you are touring, it is convenient for tying stuff to as well.
STM
Edit: Just read this post; Interesting Energy gel holder for distance riding and can see another obvious benefit 