My first cover was a thin plastic sheet that came free with a patio set, and by the second rainstorm it had pooled water in the middle and caved in on itself. The lesson was simple: a cover is only as good as the shape it holds.
These earn their place in almost any outdoor setup that gets left to the weather for more than a weekend, which is most of them. A small apartment balcony with two chairs and a side table needs this just as much as a backyard with a full dining set, because rust and mildew don't care about square footage. They matter most in places with real seasons, where furniture sits through a wet spring, bakes all summer, then gets abandoned the second the first frost hits. If you're somewhere with mild winters and you still use the furniture year round, you can get away with lighter covers you take on and off often. If you're putting things away for months at a time, you want something built to sit there through sleet and sun without quitting.
In an ordinary backyard, a good cover should disappear into the background rather than become its own eyesore. I like ones in a plain charcoal or taupe over anything shiny or branded, because a cover is there in every photo of your yard for half the year, whether you like it or not. They sit well with furniture that already has clean lines, a simple table and chair set, a bench, a fire pit ring, anything where the cover's shape just traces the object underneath instead of billowing around it like a tent. Covers with a cinch cord at the bottom or built in vents make a real difference, since trapped moisture is worse than no cover at all. I learned that one the hard way with a cushion box that grew something I'd rather not describe.
Measuring is where most people, myself included, go wrong. Measure the furniture's actual height, width and depth at its widest points, not the box it came in or a vague guess, and add a few inches of slack so the cover isn't stretched tight like a drumskin, which is how seams split in the first place. Round up if you're between sizes, since a cover that's slightly loose still sheds water fine, but one that's too tight will crack at the folds the first cold snap it hits. Also check the drop, the part that hangs down the sides, because a cover that stops short of the ground lets wind whip underneath and undoes the whole job.
Measure with the cushions and any attachments still in place, since that changes the real height more than people expect.