Corridors of attack are better sealed off with a limited static defense composed of lower cost assets than are commonly found manevering in an overwatch.
An overwatch is a moving maneuver designed to cover terrain with minimum loss of men and materials. The overwatch element is not static, it's a moving element, not to be confused with a supported attack. You will note that JasonC correctly states that the overwatch converts into a supported defense, attack or withdrawal, upon enemy contact.
The purpose of the bounding or alternate overwatch is to avoid or limit early contact with the enemy by making contact forward of the FEBA (forward edge of the battle/battlefield) too costly for the enemy.
It's also different, but often confused with, a recon by force. A recon by force is designed to fix the enemy by threat of attack and hamper the enemies ability to gather information on the main element's intent, forces and lager. The purpose is to blind and confuse the enemy. The present CM serie's scope is generally too limited to effectively allow for effective counter-intelligence tactics.