There is no road tax, only vehicle excise duty which bicyclists dont pay. It goes straight into the treasury so it's not even really "used" per se it's just a way of exerting downward pressure on inflation (which is how it should be, IMHO).
The reality is probably that it's not that well coupled to any cause, and really it's just a way to farm money from the majority of the country, who need to use motor transport to live their lives. The advertised political reason will change depending on the concerns of the populace at the time.