What is jealousy good for?

I am talking about the jealousy one can feel when being in a relationship.
Since i am in one again, for about a year now, i’ve experienced it on some occasions.
There were pretty meaningless causes, and that is exactly where the reason for me asking this question comes from.
I do get feelings of jealousy, but in the same instance i can explain with my reason (and after addressing the matter with my girlfriend) that there is nothing to be worried about.
So, that is clear for me.
Except it isn’t quite.
My feelings seem to usually have the overhand for the first some hours.
The next day my reason has won it from my heart, so to say.
My question is: why do we get jealous, when in some cases there is no need to worry?
Can you give an example from your own experience and how you dealt with it?
I hope i put it clear enough.