Re-hanging a sagging run
Gutters sag when the hangers give out, and old spike-and-ferrule hangers are notorious for it. A repair pulls the run back to line and re-secures it with modern hidden hangers screwed into solid wood. If the fascia behind it is sound, this is a straightforward fix that buys years.
Re-sloping for actual drainage
Gutters need a slight, deliberate fall toward the downspouts, about a quarter inch per ten feet. When that slope is off, water pools in the middle and overflows even when the gutter is perfectly clean. Re-sloping resets the pitch so the run drains the way it should. If your clean gutters still overflow in one spot, this is usually why.
Fascia-board repair
The fascia is the wood the gutter hangs on, and years of overflow can rot it. Soft or crumbling fascia will not hold a gutter no matter how you fasten it, so it gets replaced before the gutter goes back up. Catching this at the repair stage is far cheaper than catching it after the gutter falls.
Seams, corners, and downspouts
Split seams and leaking inside corners can often be resealed or re-joined rather than replaced, and a downspout that blew off or got crushed is a simple swap. These are the small fixes that quietly prevent the big foundation problems, and no honest crew needs to turn them into a whole-house quote.
Repair or replace, the honest math
One or two isolated fixes on otherwise sound gutters: repair. Rust through the metal, seams failing all over, or a system so old that fixing one section just moves the leak to the next: replace. A straight installer will show you which you are looking at and let the gutters make the case, not the salesperson.