Originally Posted by cg1985

Visibility, Safety. People tend to take curbs they can't see around overly casually. So if they come flying around the corner, and then merge left a bit to go around you, they might hit another car coming the other direction, which can lead to injury for the drivers and the cyclist that happen to be near by.

Taking the lane is supposed to make you more visible to both sides of traffic and prevent the people behind you from making dangerous passes while you're vulnerable.