To keep the queen producing brood, to keep them producing wax, to keep them from being malnutritioned - they are reluctant to break into capped honey unless they have to.
Bees that are malnutritioned all summer long are going to have a harder time fighting off diseases/parasites AND build up a healthy population of new bees to over winter.
Unfortunately feeding does cause robbing - so does excessive exposure of open hives. Last summer it was a big problem. This summer I've kept most inspections to a quick peak under the hood unless I suspect a problem - check for visible brood and honey along the top and plenty of bees, then close it up before trouble starts. Not an ideal inspection, but Less stress on the bees and not much robbing so far.