Purpose Of ERP?

Now, I am totally lost. Nope, not because the GPS on my motorbike is not working... actually I don't have a GPS on my bike :p

Can someone remind me again on the purpose of ERP? While the heading in this ST article suggest ERP is to keep the traffic flowing, the article also quoted PM Lee saying

"When ERP was increased, we also reduced road tax and improved public transport. As a result, many more Singaporeans can now own cars. With more cars on the road, we need to increase ERP to keep traffic flowing." (ST, 17 Aug)

Forgive my ignorance but I perceive it to mean ERP increase is for the purpose of ... increasing ERP? Ok... maybe what he meant was ERP increase has created the unintended result of ERP increase. If I work backwards, the unintended ERP increase is a result of more cars on the road. Backtrack a bit more, i.e. more cars on the road is caused by... reduced road tax and improved public transport?

So what are we actually trying to achieve? Are we encouraging Singaporeans to switch to public transport? Or are we encouraging Singaporeans to own a car? My simple mind simply cannot work out the logic.