It is easier for young kids to learn to ride their bikes if their feet touch the ground, so a low seat is good.

For adults, you can maximize crank lengths, posture, and etc with a higher seat. Your leg should be more or less straight with the heel on the pedal and the pedal at about the 7:00 position. With adequate pedal clearance, it is hard to do that and get the feet to the ground.

There are crank forward (semi-recumbent) bikes with a very upright posture, and allow the legs to be extended in front rather than straight down, and thus allow reaching the ground.