In Tanking 101, If you've never played with enemy nameplates or health bars turned on, this is one of the easiest first things you can do that will enable you to tackle more than 1 enemy at a time and keep tabs on all those meanies trying to kill your party members!
Name plates are visible through walls & floors. You can actively monitor each and every one of the enemies in a group to know how close they are to being dead. It can also make targeting specific enemies at a distance a little easier, as the name plates stay the same size within their viewable range.
Even in PVP/battlegrounds nameplates can make it easier to find that darn Arcane Blast spamming mage hiding in a bush!
Addons Mentioned: Tidy Plates + Threat Plates
This addon is functional outside of tanking though, If you're in a healer or DPS role, the addons works in the reverse as described in the segment (They're green when you do not have threat, turn yellow if you're about to pull threat, and turn big & red if you're about to get the bad stuff)
(Threat plates is not a second addon, it is an optional "theme" for use with the Tidy Plates addon)