The first question is age. An architectural shingle roof in the St. Louis climate realistically lasts twenty to twenty-five years; three-tab shingles closer to fifteen. If your roof is under twelve and the problem is localized, a repair almost always makes sense. If it's over twenty, we'll be honest that a repair is buying time, not solving the problem.
The second question is how widespread the wear is. One failed pipe boot or a section of lifted shingles on the windward side is a repair. Granule loss across the whole south face, curling tabs on every slope, and soft spots in the decking are a replacement, because there's no sound roof left to tie a repair into.
The third is layers. If the roof already has two layers of shingles, code in most of the metro won't allow a third, and adding weight to old decking is a bad idea anyway. At that point any significant work becomes a tear-off.
The last question is what's underneath. We check the attic for daylight, staining, and ventilation problems, because a roof that's cooking from below will fail early no matter how good the shingles are. When we hand you the written estimate, it says which of these we found and why we recommended what we did. You should be able to read it and agree.