I had a guest touring my AP set up and asked.....' Why do I bury my pipes ?'
For a few reasons.....
First, It does protect the pvc from UV rays. Pvc will 'eventually' become brittle. I do paint what pvc is exposed to the sun. A little 'planning ahead' and it can be done 'prior' to being installed.
Electrical pvc conduit is UV protected...plumbing isn't.
Second, Since 'most' of my lines are gravity fed and /or 'water seeking it's own level'.....I am able to give the runs a slight pitch, if I feel they need it.
Third, It makes for a neater looking area, and it's safer...one less thing to be tripping over, Or guests and kids kicking something lose.
When I bury them, I'm going 'just below' the surface....less digging. Some of the tops of the pipe are actually being covered by the mulch. I don't have a 'freeze line' in central FL to be concerned with :-)
Here again, a little planning ahead...and I bury the smaller pvc deeper...less digging that way.
I do take pictures, before I cover the pipe. It's nice to be able to refer back to them, if needed.