They don't "need" one. But if done properly, it can help guide the lighting. Some styles use it, some don't. Mobile games in particular usually don't, in order to save resources. A solid example of how it can help can be found by looking at how some 2D games use normal maps.
Replies
Or
No
It depends
Who's making the artistic decisions?
A solid example of how it can help can be found by looking at how some 2D games use normal maps.