What do plants do for aphids?

Soft-bodied insects that leave their food source diseased, sap-poor, and unattractive are what aphids are and what the harm is that they do to plants.

Specifically, an aphid is a pest that preys upon plant parts. Aphids like to apply mouthparts known as stylets toward holing into a plant's foliage in order to extract phloem and xylem sap. In the process, they make a plant not only look discolored and un-green but also damaged and diseased (because of the viruses which their mouthparts and saliva vector). They additionally will encourage attacks by wildlife -- such as ants -- in search of aphid honeydew droppings or on a quest for habitat -- such as the fungi seeking to establish sooty molds on vegetation.