All bike tubes lose air pressure at a steady rate.

This is primarily due to the light weight of the tubes and tires.

Road tires should be kept up to at least 100psi, and they need to be checked before every ride as they may lose up to 5psi a day.

Hybrid type bikes vary but should be pumped up at least into the middle of the recommended range on the sidewall of the tire and should be checked every week.

Mountain bikes lose pressure more slowly due to the lower pressures but they should still be checked every week.

Most people should use at least 35psi for trail use and 50-60psi for road use.

The most important reason to keep tire pressure up is to dramatically reduce your chance of getting flat tires.

The second is to get the right balance between rolling friction, ride comfort, and traction for the given discipline.