Marginalia— notes, doodles, or other payloads placed in the margins of books or even between lines — are fascinating and it’s too bad that we stopped using them. Endnotes and footnotes require a reader to either flip to the entire end of a text or break their concentration and get down to the bottom of the page with their eyes.
Hyperlinks are a poor substitute, and even more likely to distract or derail since you have to follow the link, open a new window or tab, and then return to the original text.
A gloss is a special type of marginalia, designed to define a word or add meaning to a phrase that might be unfamiliar. Glosses were written either between the lines of the main text or in the margin at the same level as the unfamiliar term, or with one leading into the other.
The ability to keep your place in the text while getting a semantic rundown of an unfamiliar term serves a recognizable purpose, even today 800+ years after this text was produced.