No, it's because it's a stronger grip. You can hold drastically more weight, and lift drastically heavier with that grip (or hook grip). With double overhand, the bar rolls out of your hands, backwards towards you. With the alternate/mixed grip, the bar is rolling from one hand to the other hand, enabling you to hold significantly more.
Usually you do all or most of your warmups with double overhand, then for your last warmup rep and for your work set you switch to mixed grip or hook grip. Do not do your worksets with double overhand grip, you're dramatically short changing your deadlift sessions.