The blue line-in jack on a sound card allows for external audio devices to be connected. Cassette player, CD player, audio mixer and additional microphones are types of external audio devices.
The green jack allows for speakers, the red jack is intended for microphones and the blue jack is for any other type of input that the user might want to connect to the computer. An electric guitar could also be connected. This jack is helpful for digitizing older types of music, such as records or cassettes, or for recording music. It also allows users to utilize computer speakers as a home audio system.