Transistor in saturation mode?

I was wondering why do we so often want out transistor to work in saturation mode? Ie. For a MOS Current Steering Circuit we want our transistors to operate in the saturation region....

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.