What are we doing?
We will be using a theory of experience that has evolved in language to reconstrue certain biological phenomena.
Why use a theory of experience evolved in natural language?
On the SFL model, scientific theories are higher-level semiotics realised in registers of language and the semiotic systems that language makes possible (Halliday & Matthiessen 1999: 605-6).
And not forgetting … Orgel's Second Rule: Evolution is cleverer than you are.