A blocked gutter does not just overflow. It holds standing water against the fascia, wicks moisture back under the first course of shingles, and in winter it builds the ice dam that lifts them.
Where the damage shows up
Two cleans a year, spring and late autumn, prevent almost all of it. Skip them and the damage turns up in predictable places:
Rotten fascia boards behind the gutter line
A soft edge along the eaves where water has wicked back under the shingles
Ice dams in winter that lift the first course
Standing water sitting against the wall below
Sizing matters as well. A steep roof with a large surface area sheds more water per minute than a standard gutter can carry.
Guards are not a substitute for cleaning
Mesh guards keep leaves out and they are worth fitting, but they do not keep shingle grit, pine needles or roof moss out. Those wash straight through and settle in the bottom of the run, and because the guard hides them you stop checking.
Fit guards if you have trees, then still look once a year. The point of a guard is to turn two cleans into one, not to end them.
Two things worth checking while you are up there
While you are up there, check that downpipes actually discharge away from the foundation rather than straight onto the soil beside the wall.
If your gutters were fitted at the same time as the last roof, budget to replace them alongside the next one. Hanging new shingles off tired guttering is a false economy.