It's not used to pay salaries, fund stuff etc. The govt can pay for that stuff even if it were to collect no taxes. Why? The same reason that the government can run on a deficit. It can print it's own money. The govt can press a keystroke on the computer and make an entry on a balance sheet.
Ultimately, fiat currency is an IOU that the govt issues. The $5 is a promise issued by the govt that you can redeem this piece of paper for a certain value of goods. When you pay taxes, the govt is holding it's own IOU.
Imagine you wrote an IOU to someone for $5 and then later they gave you back that IOU. How much is your own IOU written in your own handwriting worth to you? Nothing. You can write as many as you like. It's only worth something when it's a liability that someone else holds. When you hold it, the IOU isn't money any more. It's been removed from your IOU economy. LIkewise, when the govt gets your tax money it gets its own IOU's back. It gets removed from the economy.
When a country issues it's own currency, there is no need for tax revenue to pay for expenditures. There isn't a balance sheet with tax on one side and expenditure on the other side. That is very different from a membership club (an example used earlier in the thread) or even a state govt. The govt destroys money when it taxes and creates money when it spends but there is no dependency between one and the other.
Why do we pay taxes if we don't need to?
(1) as an instrument of fiscal policy to help stabilize the purchasing power of the dollar;
(2) to express public policy in the distribution of wealth and of income as in the case of the progressive income and estate taxes;
(3) to express public policy in subsidizing or in penalizing various industries and economic groups; and
(4) to isolate and assess directly the costs of certain national benefits, such as highways and social security.
I sort of look at tax money the way I look at the bill at a restaurant after I had a meal. Before the meal it was my money, after dinner it belongs to the restaurant.
Not that I would have spent the money the same way but alas the people voted to represent me and us did.