[Raistlin]

A few uses explained (there are more, as some are discovering):

Sorting units within your base

If you set a location around your unit production buildings which commands all units of a certain type to move to one spot, and then set a location in the same area which commands all units of a different type to move to another spot, your units will be sorted on the fly as they are produced.

Setting attack-rally-points

The same as above, but the command you associate with the location is to attack to a particular spot on the map as opposed to moving to another spot within your base.

Staging units for an attack

Staging units is the act of positioning them relative to each other in a way that optimizes their effectiveness in battle. A basic example is putting zealots in front of goons before sending your attack.

If you set a location in the middle of the map that command units of a certain type to move to a spot outside your opponent's base, and then set another location in the same area that commands units of another type to move to a second spot outside your opponent's base (near the first spot, the two spots being positioned relative to each other so that units being sent to them will be properly staged to attack), sending your units into the location area you set will cause them move to the perimeter of your opponent's base and be staged to attack automatically. You can do this with as many unit types as you like to automate the staging of complex attacks. It is also worth noting that the locations you set remain until you deactivate them, so you can keep using them to stage your troops as you attack repeatedly.

Advanced patrols

If you set several locations around your base (or any patrol path) which command units to attack-move, and the attack-move command of each location sends the units into the next location, your units will attack in a loop (or whatever pattern you create).

Orchestrated attacks

I'm not going to spell out my attacks, but in general, you can use this to trigger your various units to automatically do the things you want them to do, by choosing a spot somewhere on the map and setting locations there which issue commands to your units. Then when you are ready to go you can just send your units into that spot and they will proceed on from there, carrying out separate actions without needing to be controlled.

Orchestrated defense

The same as above except the purpose is defense rather than offense. You can choose a spot somewhere in your base (this works best in fmp where the bases are large) and set locations there which command your various units to defend your choke in a coordinated fashion.