Are Basic Lands (Mana) Block/Set Agnostic?
You don't need to buy new ones just to stay legal.  You can use any card that has been reprinted in a recent set in Standard.  All the basic lands are printed in just about every set.  

For Instance, it's perfectly legal for you to use your Urza's Saga Rancors or Onslaught Pacifisms.

Secondly, some people like collecting different land arts, so they produce lands with new arts every set, and many people try to collect one or more of each.