Not really sure what people mean when they mention that.

This is your life, and it's ending one minute at a time -Tyler Durden

it puts a second HP bar on your screen so that you can see the HP and status effects on that target even though you are not actively engaged to it.especially good for healers to put on the mob, even though healers are usually targeted to party members.and you can make focus target macros. so you can cast Cure on the tank and still put Virus on the mob.

It's also good for DD's when fighting bosses that have adds. Example focus target Hydra so you can always see when Hydra is going to cast Fear if you're to busy killing the wyvern and not notice that Hydra moved to the middle.