I remember the B200s I once owned had that Unswitched electrical outlet, too. I had no need to use it, but it was nice that it was there should I ever wanted to use it.
For those wondering, an "Unswitched" outlet is an outlet that is powered when the unit has power to its own Mains regardless of whether the unit is switched On or Off.
A "Switched" outlet means the outlet is only powered when the unit itself is switched On.
One scenario for using this is that the preamp could be plugged into this outlet to allow proper sequence of powering up and down the preamp and poweramp if using a power strip or rack power supply (Power On: preamp first, poweramp last. Power Off: poweramp first, preamp last.) since a lot of preamps have their power switch on the back of the unit where it's not so convenient. Here you'd leave the preamp switch On, and power up the rack with the main switch on the rack power, then reach down to toggle the poweramp switch ON. Reverse the sequence when powering down the rack. There's more ways to utilize the "unswitched" outlet on the back of the B200s (and others) but this is just one.