I'm just about to finish my first trip out in the black, and I had kind of the same question on the way: when and why would I want to land on a planet.
As far as safety goes, I've been advised to avoid any planet with gravity over 2G, until I get better used to landings and do some tests in the bubble, with nothing much to lose if I slam to the ground. I followed this advise and everything was fine.
Now, about the reasons for landing, there would be 3...maybe 4:
- looking for FSD and AFMU materials, as someone pointed above;
- scenery pictures and selfies;
- hoping to find something, like Points Of Interest (POI) or alien traces and things like that; this is though a very long shot, as such things are very rarely found outside the bubble;
- I've read somewhere that driving around in your SRV should also slightly add to your explorer rank; didn't actually test this, but if it's true and you're looking for rank improvement, it can be a reason.
Honestly, in about 35k ly (my whole distance forth and back) I didn't NEED to land. And I've even used neutron stars to boost up quite regularly, which did bust up my ship on occasions. Every time I landed was because I needed to break the routine, as 'disorganize' pointed out.