When a homeowner finds water on the ceiling, they grab a phone and type "roofer near me" or "roof leak repair" plus their town. They call two or three of the first names they see, and the roofer who answers and shows up first usually gets the job. For a full replacement they take longer: they read your reviews, look at your photos and compare your site with the next one.
Those first names come from three places on the page: the paid ads at the top, the Map Pack with three local roofers, and the regular results under it. Roofing SEO is the work that gets you into the last two and keeps you there without paying for every click.
If you're not there, the work still exists. It goes to the roofer with forty more reviews, a filled-out Google Business Profile and a page for the exact job the homeowner typed. Most of the time that roofer isn't better at roofing. They're just easier to find.