h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

It is a little-known fact, possibly because of the efficacy of the symbiotic relationship built up over time which leads all life-forms equipped with aural cavities to take it for granted, that the Babel Fish produces secretions which harmlessly dissolve the ear-wax produced by the host. (regardless of species or indeed of the species-specific type of secretion, up to and including the markedly unpleasant Vogon variety, believed to have been evolved as a defence against listening to their own poetry)

This has the benefit for the Babel fish of providing it with a clean living environment, and also removes impediments to its receiving the sonic and neural transmissions on which it depends for sustenance.

The benefit for the host is an aural system which is free of nagging little impediments and an unsightly and possibly painful build up of waxy secretions inside the relevant orifice.