Well, in my opinion (and I think most other people's), the default interface doesn't give enough information.
Healers use addons that make it more obvious when someone has a debuff that they can cure.
Crowd Control classes use addons that tell them when their spells have been broken or worn off.
Tanks (especially, but all classes) use Threat Meters to tell them when someone is about to steal threat (or when they should ease up on the DPS to avoid getting aggro)
Thats just a few examples, there are many more that help raiders have more control over whats going on.
But mostly I think, the reason that most raid videos have custom interfaces, is that interface modders just like showing off
