Why one setting carries this much
Google keeps a fixed list of categories, and your primary one puts your listing in a bucket. When somebody searches for that kind of work near them, the businesses in that bucket are the ones Google is choosing from. A business in the wrong bucket isn't fourth on the list, it isn't on the list.
That's why it beats almost everything else you can do on the profile. Most settings nudge; this one decides whether you're in the running.
Picking the primary one
Pick the category that names the work you most want the phone to ring for, not the broadest one that covers everything you'll do. A general contractor category on a business that makes its money on roofs puts you against every remodeler in the county, for searches you don't want.
Type the job into Google the way a customer would and look at who comes up in the map. Their categories are visible on their listings, and the pattern across the top few is usually the answer.
If you change it later, change it once and leave it. Flipping the primary category back and forth gives Google a moving target and gives you nothing.
The extra categories
You can add more categories under the primary one, and they widen what you're eligible for. Add the ones you'd genuinely take the job for, and stop. A pile of categories you don't really serve brings calls you have to turn down.
There's a separate list on the profile for services, which is different from categories and usually left half empty. Filling it out completely is the one profile field with a repeated test behind it, and it takes about twenty minutes.
What the category can't do
It can't put you in a town you don't serve, and it can't stand in for a page. The category gets you into the pool for that kind of work; what your website says about the job and the town is what decides whether you come out of it.
It also can't be bought. Google says in writing that there's no way to request or pay for a better local ranking, so anyone selling category tricks is selling a setting you can change yourself, for free, this afternoon.