Traditional v. Natural Burial
Caitlin Doughty of Order of the Good Death discusses the differences between natural & traditional (which has only been 'traditional' for less than 200 years) burial.
Seriously, the burial vault ($$$$) was designed to thwart grave robbing but since the average criminal is too lazy to dig up a grave nowadays, that ain't a problem. Now the vault is indeed used to keep the lawns looking nice.
Well, that & so you don't hit soft spots while strolling through the boneyard. That's always a little disconcerting - especially when your foot gets stuck in a hole.