Raids - the possibility to participate in any raid of any difficulty, get the gear that you need or items for transmogrification and other rewards such as achievements and mounts. And have chance to see the story hidden in the depths of each raid!
Raids are designed as activities for people whose characters have reached the maximum level. As the game does not permit further increase of skills, or introduce new abilities, the only avenues remaining to enhance a character lay in better gear and other rewards only found in raid instances. Almost all raid events and bosses require that the raid members have particular levels of gear and skill, expressed as dps, damage mitigation, and/or healing ability. In addition, a majority of such bosses also require the group use particular, sometimes unique tactics. While occasionally those tactics mirror those used by smaller groups, they often require things that smaller groups simply don't have the diversity to do.
Given these unique requirements, most raid boss encounters require some amount of practice to defeat. Sometimes the peculiarities of a boss encounter are such that being vastly overgeared/overlevel will not permit the raid to ignore the designed tactics.