The idea behind it is rather simple: supply and demand. When there is a limited supply, you can affect demand- In this case, reduce it - by increasing price. Fewer people on the road means less congestion and everyone gets to where they are going more quickly. Charge a high enough price and some travel will be delayed to non-peak times while other travel will simply be deemed unnecessary. Some may choose to carpool or take public transportation, further reducing congestion.
It provides economic choice. Time is money. (Or is it the other way around?) When I travel between, say, Bellevue and Kirkland, I can drive, Uber (is that a verb now?), or take the bus. All have time, cost, and convenience trade offs. That principal is now being applied to highway driving.
Is it working? I don't know.
First of all, the market principle isn't being applied to all lanes - only two - so it's an imperfect market. Still, the theory is that you can move the maximum number of cars at 45 mph during peak travel times. By charging a low enough toll, you can divert enough cars to the carpool lanes to make the free ones less congested and charging a high enough toll ensures the carpool lanes don't get bogged down.
Although I wasn't around when the tolls hit $10, my experience has been the carpool lanes are generally empty while the traffic that could be occupying five lanes are now slogging through three.
Plus, just because you can charge more for a thing doesn't necessarily mean you should. The price of certain medications has skyrocketed. It might be sound economic theory but is it a good idea?
The toll did get to $10 so there must've been a whole bunch of people using it at one point. And, there's economic theory to suggest it should be even higher although the theory put forward is based on the ability to pay rather than on whether it reduced congestion.
I don't have any answers here so I apologize if you think I was going to provide a neat solution. It's an interesting question, though - how do we all get from here to there most efficiently? For now, all you can do is pick a lane and try to remember to wave to your fellow motorists with all five fingers.
While you're stuck in traffic (as a passenger, of course) consider the following additional reading:
(Posted from an Amtrak train.)