History in linguistic description

It should never be necessary to ‘justify’ a particular kind of study; a subject is intrinsically interesting and so worth pursuing, or it's not. But it is sometimes worthwhile to show why a particular study is useful or informative or ‘relevant’ (to use a piece of debased coinage) in reference to some other, where it fits in the larger picture.

Any serious student of Old English is presumably engaged in this pursuit because it somehow appeals; and anybody interested in the subsequent history of English of course needs to know Old English, since it is the input to that history. Crudely, the plural of mouse is mice because OE mūs had the nom/acc plural mȳs, and OE /y:/ comes down into modern English as /ai/.

Knowledge of antecedents provides a special son of understanding, different from the kind that comes from knowledge of structure: what we might call genetic or genealogical understanding. Historically evolved systems are not continually made afresh; they contain remnants of earlier stages, sometimes fully functional, sometimes reduced and functionless, sometimes just marginal. Understanding a system involves knowing where these things come from and what they used to do, how their current functions (if any) relate to their old ones. To take an example from another field, we all know that there are three little bones in the middle ear that transmit sound from the eardrum to the cochlea, and we can understand easily how they work and what they do.