The ridge keeps the mulch in place, and looks nice. Also, digging a well/ridge around your tree and mulching helps it to retain water. If this is a concern, I suggest doing slow, deep drip irrigation on your tree. That's what it needs to develop strong, deep roots. In drier climates the well/mulch combo helps retain moisture. If your climate is not particularly dry be sure to move the mulch around so it doesn't rot or become compressed.
