Part 1, You make dmg from mobs deadly if you are not equipped to tank. (IE give them higher levels of mitigation)
Part 2, You make DPS partners in reducing that damage. Be it with debuffs or CC (gasp)
Part 3, You make tank holding enmity much easier, and you give tanks debuffs on mobs to make DPS work better
Part 4, You Make healing work better the long you are 'on target', and make dealing damage (as healer) easier the longer you are 'on target' So that switching targets (to heal a dummy that pulled) more difficult.


DPS cannot aggro because they will die quickly. DPS shoulder responsibility for keeping tank alive, Tank is responsible for making DPS effective, healer can alternate between objectives.