Ideally this would always be the case, and in most cases it is since bikes have a pretty wide range of gears available. But two cases come to mind where there are limitations to the gearing and in those cases it can be beneficial to adjust the crank length to compensate.

The first has already been mentioned above: a small-wheeled bike may not have sufficiently high gears available to reach a reasonable top speed at a comfortable cadence. In that case reducing the crank length can allow the rider to comfortably pedal at a higher cadence than he/she could otherwise and thus partially overcome the limitation of a low top gear. OTOH, many small-wheeled bikes manage to have pretty standard gear ratios through other means. E.g. my Bike Friday uses a 60t chainring and 11t small cog to get about the same gearing as my Cannondale gets with 52t in front and 13t in back.

The second is in the case of a traditional tandem where both partners must pedal at the same cadence. Usually that works ok, but sometimes one member of the pair prefers to spin in a lower gear while the other one has a masher-style, preferring a higher gear at a low cadence. Giving this second person a shorter crank will make it easier for them to spin at a higher cadence and give them the added resistance force they're used to feeling. Then they can both comfortably pedal at the same cadence despite their different preferred styles. This assumes you want the two cyclists to remain synchronized since otherwise the timing-chain rings can be made different sizes to allow for a different cadence for the front vs. rear rider. I did do this when I had the tandem set up for my daughter as a 'kid-back'. Her cranks were way up high so pedal strike wasn't an issue and her weight was small enough that having her pedaling out-of-synch with mine was also fine. Wouldn't recommend that arrangement for two adult cyclists though unless the bike is setup for independent coasting and shifting.

Most people are pretty adaptable to a range of crank lengths as long as it doesn't get too far away from their optimum. On my bikes I have 165 mm cranks on the tandem, 170 mm on the Friday, and 175 mm on the Cannondale. They all feel comfortable, but I do notice that I tend to spin faster on the tandem and use a bit higher gearing at lower cadence on the Cannondale.