The snapshot feature is most useful when you want to preserve the state (existing state) of the virtual machine so you can return to the same state repeatedly.
While Updating the Patches recommend is need to take a snapshot of the vm.
EX:- Os level updates,Service Packs,Application level updates,EXchange 2007 sp1,2..,.Net framework 3.1 sp1 …etc.,

Snapshot Files as below

VMname.vmsd = VM snapshot list
VMname-snapshotname.vmsn = VM snapshot file
VMname-Delta.vmdk = VM snapshot data

Here, Delta = Any constant value = 0001,0002….etc.,

Snapshot is helpful when you revert to the same virtual machine state instead creating multiple virtual machines.