The reason why the concept is important in the context of the Dynamic Energy Budget (DEB) theory is that food (substrate) uptake is proportional to surface area, and maintenance to volume.
Since volume grows faster than surface area, this controls the ultimate size of the organism.
Alfred Russel Wallace wrote this in a letter to E. B. Poulton in 1865.
The DEB theory explains why isomorphs grow according to the von Bertalanffy curve if food availability is constant.
Most animals approximate isomorphy, but plants in a vegetation typically start as V1-morphs, then convert to isomorphs, and end up as V0-morphs (if neighbouring plants affect their uptake).