Hi Jim,
This is helpful if you have a shortage of interfaces. If you have two interfaces and you know you're not going to need spares, then you can do either way and they are equivalent.
They are more often used in situations that:
1) You have more vlans than ports
2) You want to leave the other port to do other things, or for expansion.
They're really quite simple, and once you get used to them you may find them easier. Because if in the future you want to add another VLAN/IP to the router, it's as simple as adding another subinterface and there are no troubles.
hth,
nick