Transistor in saturation mode?
FETs in saturation are typically used to obtain a constant current. If you look at the I-V plot for a FET, the lines are relatively flat in the saturation region. For a given \$V_{GS}\$ curve, the current will be basically constant over a large range of \$V_{DS}\$. This is what we want from a current source; the flatter the slope, the larger the output impedance.