For plugging in "Line Level" instruments such as Keyboards, samplers or drum machines. They can also be used to accept the returns from multitrack tape machines and other recording media.

Keyboards, Drum Machines, CD Players, DAT Machines, Wireless Mic Receivers, all provide line level outputs. If some of your instruments are STEREO connect their left and right outputs to a spare stereo input. Alternatively connect to an adjacent pair of mono
inputs and Pan the inputs hard left and right to create a stereo image.