its how big the clips are going to be saved, it cant record all day into a single file due to fat32 filesystem restrictions (2GB file limit).. if you go to download a a clip from the recordings thats how much time it will take.. do you want 12 1h files stored each day? if you need to save a 10s clip your going to have a big file to archive.
on the inverse, if you make them too short then you have to merge them together for longer activity or anything spanning clips.. I find 15-20min clips is pretty good for continuous recording.