Well, photos and videos can take up a lot of space. A lot. So having a decent sized SD card and keeping them on there rather than the phone itself is a way to save space. And while you can't move most apps, there will be some here and there you can move to the card as they don't requite "Internal" storage.
Just be careful to back things up to a better location like a computer or cloud account.It is way to easy to have a device die, lose it, have it stolen, get damaged (same all goes for a SD card).