Exactly the same as every other tax you pay. It all goes into "Consolidated Revenue"- income tax, petrol excise, cigarette taxes, alcohol excise etc. etc.- pretty much every dollar that is collected by the government from the citizens- and from this pot, things are paid for such as roads, schools, hospitals and so on.
There is nothing special about petrol taxes- it's just another way for the government to pull money from your pocket in order to fund the things we all use. In reality, anyone who pays taxes of any sort has a right to use the roads- which is pretty much everybody. Cyclists included.
