Why do companies change their logo/wordmark (e.g. StackOverflow)?

[profsummergig]

StackOverflow had a nice logo.

To me, their new logo seems worse. (To me, it's an uglier version of the original logo).

Why do companies change their logo?

[SanjayMehta]

New management has to assert themselves.

This is the easiest way to get visibility. Have seen it a couple of times. Whether it helps the company or not, I'll let the marketing folks answer.