[sammyg2]

The whole reason for disconnecting the O2 sensor (besides those who don't want to fix something that is broken) is to set the mixture the way you want it, instead of the way the O2 sensor wants it.

Some folks like the little bit of extra torque available from a mixture just on the rich side of stroichiometric and are not concerned that much with maximizing economy or minimizing emmisions.
I prefer to have a fully functioning O2 sensor, some others don't.