Making it more scannable and searchable. They’re looking for keywords, probably using a software routine. Make sure you know what they are, and use them. You may get no more than a fast look as a first cut, so you have to get sorted in, not out.
How do you present yourself in your whole, intact resume? and how well do your qualifications fit a profile of an opening? Those are two different things. For example, being able to fill in a certain field gives no indication of whether you can write a coherent sentence. In some jobs, that actually matters.
Maybe also looking for possible fit with other openings if you don’t score a hit on the one you’re responding to.
Also shedding special fonts, weird characters, and other typographic and formatting tricks that applicants may use (the way they used to use deckle-edged creamy rag bond) to try to get attention.
And, of course, better to waste your time than theirs. Why should they devote resources to considering you if you can’t be bothered to comply with their requirements?