There's an interesting motivation in the paper by G. Segal: 

*K-homology theory and algebraic K-theory.* K-theory and operator algebras (Proc. Conf., Univ. Georgia, Athens, Ga., 1975), pp. 113–127. Lecture Notes in Math., Vol. 575, Springer, Berlin, 1977

It is well-known that the space of Fredholm operators gives a model for
**topological** K-theory $\Bbb Z \times BU$. 


According to Segal, if $C$ is an exact category, a morphisms of the Q-construction $QC$ of is supposed to something like a generalized Fredholm operator. 

So algebraic K-theory via the Q-construction mimics topological K-theory defined via Fredholm operators.