Have you ever seen one of your teammates die because you couldn't realize fast enough he was the one the boss was suddenly hitting?
Have you ever faced a mob switching aggro and making almost impossible for you to follow his hits with your heals?
... and if that never happened to you, then be SURE it will happen soon.
In fact, at high levels, you NEED an (almost) automatic way to heal the right person, ther current target of a mob. If not, you let your teammates die sooner or later.
The answer is called "assist heal". Basically you assist the mob to have the same target as the mob, and, thus, heal that target!
The explanations are there in a great assist guide by Doctorhyde:
http://www.athenpaladins.org/forums/viewtopic.php?t=672
This is the part about docs, but you'll need to read the beginning of the guide too.
Doctorhyde wrote: GETTING DEEPER: ASSIST HEALING
The REAL power of assisting for healers comes in combining both of the above macros. Most times in a raid with multiple teams, there will be one or two tanks taking the majority of the damage, but aggro can shift quickly. So we need a way to be able to quickly see who is taking damage right now. (...)