Since the advent of Microsoft Word, the use of proofreading marks in editing is rare. Fond remembrances of the squiggly lines lingers as people seek nostalgia of writing’s past.
Decades ago it was tedious to move paragraphs, sentences and words around because most people used a typewriter. All of this had to be done with a paper and pen with a sometimes unreadable piece of paper with stray marks such as arrows and crossed out sentences.
The marks were supposed to be legible so that you or someone else could type them up as stated in this government manual: “Instructions and marks should be legible, intelligible, and close to the change desired. Avoid marginal notes that dogleg around the page and “skyrockets” that zoom a change from text to a distant part of the page” (Proofreading, p. 266).
My how things have changed since then. Being in the writing/publication business, I frequently work with editors. Just in case if you wondered how the process of marking up copy works, I’ve pasted an example below.