I think it was to pay for the bridge. Spokane is over the WA line from us so I'm not 100% sure.
Most bridge tolls are implemented as a way to pay for the bridge. At least in the SF Bay Area, the tolls were meant to be temporary to pay the bridges off...but, by the time the bridges were paid off, they needed earthquake retrofit work, so the tolls were kept in place (and even increased a few times). Now, the tolls are in place for two reasons: 1) to pay for the new Bay Bridge eastern span, and 2) because the rest of the state (L.A., the San Joaquin Valley, etc) said that they weren't willing to help pay for upkeep for pieces of infrastructure that weren't in their area.
In the days of electronic toll-taking, I just wish they'd implement high-speed toll lanes statewide. I've seen it on a few bridges, and it really works better than having to slow to 10 mph or so to keep from smacking a mirror against one side or the other. It might not be a huge deal in a passenger car, but in a bus, it can really be tantamount to threading a needle. I think every mirror on every one of my company's buses has damage from hitting the side of a toll booth
(you only have about six inches of clearance on either side)