The idea that friendship is based in some fundamental way on usefulness may seem surprising if we imagine Plato's Socrates to be a philosopher solely of pure knowledge and virtue. But usefulness often comes into play in the dialogues as a mediator between the abstract virtues that Socrates attempts to construct and the worldly, practical virtues assumed by his interlocutors. This is certainly true in Lysis, where Socrates has to convince two energetic young boys that knowledge and friendship should supersede goals like owning dogs and horses or driving the family chariot. Thus, Socrates convinces Lysis to strive for knowledge and understanding because these things will make him useful, and so give him greater control over the practical (and fun) elements of his life.

In the arguments about friendship, however, use-value seems to play a deeper role, functioning at the very root of love and desire. Specifically, the demand that two friends be "useful" to each other prevents any possibility that likeness can be the cause of friendship (since two people can get nothing from each other to the exact extent that they are the same). Although the various possible causes of friendship include a wide range of qualities over the course of the dialogue, most of them depend at some point on this notion of practical use, a notion that friendship is to some extent a profitable exchange (as with the sick body "befriending" medicine).