Is it necessary, feed?

Is giving a horse sweet feed all year round necessary? We generally only give it to ours in the winter or if they have been worked (which due to lack of time or places to ride isn't all that often). They are already fat from stuffing themselves on gr and hay (there is usually a round bale out most of the year), but a friend of mine said that they need sweet feed. Do they really need it in the summer or just the winter?