Like other major 3D softwares, blender has come up with Modifier stack. Now the sub surface is available under Modifier which gives the flexibility of using it with relative to other modifiers like wave, build, etc. Modifier helps to easly make changes with visual preview and apply changes to the mesh with just a mouse click. The most happiest thing is, even if you applied a modifier to the mesh, you can make Undo. Of course before doing any other changes to the mesh ;-) .