Here, all the CTA buses have the racks as well. I’m not sure why, as it’s pretty easy to outpace a bus on your bicycle most of the time, since it’s pretty flat here. I suppose when it’s raining, it might be useful.

There’s some information on the CTA’s program in their Bike & Ride brochure.
In Seattle, you could check out the
metro bus website, for details on your particular rack (which appears to be the same as Chicago’s.)