Two people are doing a fight. Person 1 does 200K single target DPS, or 50K per target AoE. Person 2 does 100K single target DPS, or 80K per target AoE. They're fighting a boss with 19M health, plus 4 adds with 4M health each.
Person 1 wants to AoE, right? He does 200K single target, or 250K AoE (5 targets total), so obviously he AoEs, surely? Well, no. Not if he's trying to maximise the raid DPS instead of his own DPS.
If both players AoE, their combined 125K per target DPS kills all the adds in 32 seconds; it takes them 50 seconds more to finish off the boss. (15M health left on the boss after AoE phase is ended, 300K combined DPS.)
If player 1 ignores the adds and just single-targets the boss, leaving player 2 to handle the adds on his own, it takes 53 and a bit seconds to kill the adds instead of 32 - but by the time the adds are dead, the boss almost is too, and it only takes another 14 and a bit seconds to finish off the boss too, for a total fight time of just under 68 seconds instead of 82 seconds.
How does this work? In choosing to AoE, both players would sacrifice boss DPS to increase their total DPS - but player 1 sacrifices a lot to gain a little, while player 2 sacrifices a little to gain a lot. It's beneficial for the raid if player 2 can do that as much as possible. So in choosing to single target, although player 1 has given up some of his potential personal DPS (and not even much - he would have averaged 219.5K in the first scenario, vs 200K in the second), he's allowed player 2 to significantly increase his. With the adds lasting longer, player 2 averages 316.4K DPS in the second scenario, vs 207.3K DPS in the first. Each player focusing on their strengths makes the whole thing go faster. Do make sure you think about yours in a given fight.
Important caveat, of course: the adds are alive for longer in the second scenario. This may or may not matter. If it matters, and they need to die ASAP, everyone should switch regardless of cost.