A diagram has the obvious advantage of presenting things visually by means of arrows and boxes, and showing relations at a glance. A discursive presentation tends to be better at suggesting temporality, nuance and development.
The difference is not without significance in this case. Holmes began his essay by drawing analogies with the natural sciences and then spoke of the study of translation as having a theoretical and a descriptive branch. This echoed the division between theoretical and empirical work in the sciences. Only after a thorough discussion of these two branches did Holmes mention that there was also an applied branch of translation studies.
At the time Holmes wrote his essay, most of the work in translation studies was of the applied kind. Holmes’s privileging of theoretical and descriptive work can be read as promoting the view that these ‘pure’ kinds of research, rather than applied studies, should be recognized as the core of the discipline.