Jealousy in relationships is kinda like cinnamon on a snickerdoodle: without any, you've got no flavor at all; a little taste of it can spice things up; but any more than just a pinch and you might just choke to death.
In tiny doses – say, when you take note of a stranger desperately attempting to hit on your partner to no avail -- jealousy can function as a reminder of how desirable your companion truly is. As long as you've built a strong foundation together, tiny flashes of jealousy here and there should be harmless at worst -- at best, they can be harnessed to bring a little excitement to the relationship. Hell, some couples even take it further with various kink scenarios, such as in cuckold roleplay.