Simply put, behaviour that provokes laughter or provides amusement can be considered humerous (English versus American spelling).

Typically humour creates good feelings.  Try laughing and being angry or sad at the same time. It is possible but hard to do and usually the humour would at worst temper the angry or sad feelings.

Sending. message with Humour

Using humour to send a message might reduce the clarity and impact of the message; however it undoubtedly increases receptivity.

Hiding a message within a humerous interaction can often cause it to be missed or downplayed .  This can be both an advantage (the message is softened so receptivity increased) and a disadvantage in that the message could be completely ignored and have no impact.

Use with Care

Highlighting a behaviour you would like to see modified in a light and airy way is easier than the direct and difficult conversation.  I often see less effective leaders/managers opt for this approach.

They are uncomfortable with the difficult discussion and so avoid it. Humour lets them get the message out and save themselves some stress.

As I have noted here , there are a lot of good reasons to use humour to effect behaviour change, unfortunately this is not one of them.  It can often result in the situation not being resolved.

Since the direct approach is shunned,  in these cases the issue often remains unresolved.  Additionally the manager/leader might try to use the humour repeatedly.  It did not work at first so try again.

Funny thing about humour though, on repeated use it often morphs into something offensive.  At that point the chances of eliciting positive change become severly diminished.