They are peacemakers between the hens, or can be. I've seen several of mine break up hen fights by literally jumping between them. They are much more apt to see trouble than the hens are, in my experience, however, sometimes I think that hens with a rooster become a bit lazy at watching out for themselves. Roosters will watch the skies and area about 80% of the time, while the hens spend about that same amount of time stuffing their faces with their butts to the sky, LOL.
 
They can develop a dislike for a particular hen; I know my own Delaware rooster, who is really awesome and my good buddy, doesn't like the barred hens, especially one of them, but I keep him because he is completely non-human-aggressive. I'll be adding one or two from chicks I'm raising to his flock for back up (not Delawares, who are too motivated to breed-Isaac has 24 hens to himself) while on range.
 
I have had chickens for over 6 years, live in the woods on a mtn with every predator you can imagine (except those peculiar to only the North) and have never had a predator loss, even free ranging all that time. I credit my roosters for that. There are great roosters and there are not-so-great roosters, certainly. If you get a great one, you'll be sold.
 
I don't keep roosters here who are aggressive to people, no need for that (the D'Anver bantams can be fiesty, but they weigh less than 2 lbs, LOL). They do crow even at night sometimes, but that doesn't bother me.
 
 
Here is an extreme story of how great a rooster really can be, though he is a very rare fellow: 
https://www.backyardchickens.com/t/521876/why-have-a-rooster-new-pg-10-video-pg-13