Scrolling Combat Text makes doing damage and healing fun. There's nothing more like scoring a critical hit or heal and seeing the exact amount come up floating in the form of easily visible numbers. Forget watching the combat log to see what happened; you'll be there in live focusing on your character and watching everything that happens.
World of Warcraft already has a scrolling combat text option available, but that doesn't cut it for professional WoW players that's why it is recommended you use the Scrolling Combat Text addon. This addon will show you damage, healing, identify casters of spells, show you when you resist or cause spell damage, all of your mitigation / defensive abilities like (dodge, block, parry), allow you to set custom colors for everything (it comes with a configuration file for more modification) show you debuff and buff alerts, provide you with low health and mana warnings that can even play sounds, show you mana/energy/rage gains, give entering and leaving combat alerts, provide combo point alerts (for rogues), class skill alerts, and much more.
All of this might seem like fluff, but for most of you who PvP or even PvE know that knowing things like when you are in and out of combat can make or a break a fight because of abilities that can't be used in combat, drinking and eating, or other reasons. Some people who PvP might not want to just spam keys constantly hoping that their special abilities popup, instead they can watch for Scrolling Combat Text to alert them that an ability is ready to go.
After playing with Scrolling Combat Text the game becomes more real and it's hard to play without it. If you haven't already downloaded this addon I suggest you give it a try.