Why are non-biochemical features such as explicit equations included in SBML?
The aim of SBML is to enable the construction of quantitative models that describe both the activity of biochemical networks and interaction of biochemical networks and other phenomena. SBML allows the declaration of variables (non-constant parameters) and associated ODEs and DAEs to describe these phenomena. Examples of these phenomena include the mechanical force generated by muscle cells and the electrical potential across a synapse.