Why use strings of letters instead of hypergraphs in some examples?

Strings of letters are just a simpler stand-in for teaching: a rule like BA -> AB is easy to read and to draw, while hypergraph rewriting needs genuine subgraph matching that is harder to follow by eye. [setup] The underlying ideas — applying a rule wherever it matches, the resulting causal and multiway structure — are meant to be the same for both; the string is a low-dimensional model of the same rewriting process. Hypergraphs are the real object of the theory because they can grow the higher-dimensional, space-like structure that strings cannot.