Summary of how it is used in games
There's no hard rule on creating it, other than prioritize utilizing the cheaper primitive types as much as possible and keep the object count low.
However there can be cases where a convex hull could be better than multiple box primitives (i.e. an 15 face hull could be cheaper than using 10 boxes to cover the same shape).
Why it is important
Story time: There was a rock in Mass Effect 1 where the original artist just used the auto convex hull creator common on, that resulted in it generating perhaps somewhere in the 50+ collision object count. A problem occurred if the Mako crashed into it near the end of the level on Therum, as the wheels were dynamic, and thus were constantly checking against the
Factors to consider when making it
Streaming - with huge, detailed levels, one can split the collision between overlapping objects, be wary if one part streams out and takes a chunk of the collision with it, don't want to fall through the world due to it (i.e. test with and without the other chunks loaded).