Most people buy a runner the exact size they think they need, which is to say too short, and end up with a rug that looks like it gave up halfway down the hallway.
A runner only works where the space is long and narrow, so a hallway, a galley kitchen, alongside a bed, the strip of floor beside a staircase. Put one in a square room and it reads as a mistake, like someone measured wrong, because in that context it basically is one. It also clashes badly with anything else busy underfoot, a patterned stair carpet, a tiled floor with its own strong lines, so if your hallway already has a personality, let the runner be the quiet one. And be honest about the limits here, a thin flatweave runner is not going to cushion a tile floor or muffle a creaky staircase the way a proper rug pad and a thicker rug might, it is mostly about the look and about not tracking mud straight onto wood. If sound and cushioning are the actual problem, a runner is the wrong tool and no amount of nice wool is going to fix that.
Getting the size right is where most of the damage happens. Measure the full length of the space first, then leave somewhere between six and ten inches of bare floor at each end, don't let the rug stop dead at the doorway or run wall to wall like it's trying to escape the room. Width matters just as much, you want a few inches of floor showing on each side so the runner looks placed, not wedged. In a kitchen, keep it clear of cabinet doors and the dishwasher swing, nothing worse than a rug bunching every time something opens. On a staircase, each tread needs to be fully covered with enough overhang to tuck and secure, not just a strip floating in the middle. Lay a few painter's tape lines down to mark the actual footprint before you commit, your eye lies about length far more than it lies about color.
Runners suit older houses with long entry halls, railroad apartments where every room is a corridor to the next one, and any home with a staircase that needs a bit of grip and personality. They're also forgiving through a wet, muddy season, something you can shake out or swap for a cheaper one without mourning it. A thin, flat runner earns its keep fastest in a rental, low commitment, easy to roll up and take with you.
I'd put the first one down in the hallway, every time.