They protect the aphids from harm.
Aphids are often unable to even crawl out of harm's way. There they are, sucking the life out of a plant. Sitting ducks to any ravenous meat eater.
Here's where the ants come in to the picture. Did you know that ants and wasps are somewhat related. Vicious creatures. To get at the aphid, lacewing or lady bug larvae have to get past the antz! They are all part of that balance of nature thing.
I'm willing to put my thumb on the scale. I mean, sunflowers attract aphids so well that we can use them as a trap crop. What then ... pile dry brush around them and set fire to them??
No, for me it's just like Mom said, "nothing that a little soap and water won't fix." Or, to the barricades and stymie the ants with tangle foot ... and let our friends the lady bugs and lacewings take care of the aphids.
Steve