It would seem that if the system was fixed then we wouldn't have a choice as to how we pay.
For example, your property tax assessment comes to you in the form of a bill. No voluntary compliance, just pay it or move.
Damn you always get me when I am brain dead but I will give it a shot. The W4 is to calculate your "dependent or implied dependent" deductions. In other words you may have only yourself as a deduction but your other "implied dependents" can be as many as 4 or 5. That means that your paycheck tax deductions are greatly reduced and then when you "voluntarily" file your return your tax withholding balances with the deductions you have claimed on your W4. For example I had 4 kids a wife and myself as flesh in blood dependents but for years I carried 11 dependents on my W4 based on interest write-offs and business expenses I incurred during the year. My actual withholding was almost 0 but I never got a refund either. The upside is that I kept my money and the downside was that I sometimes had to pay at the end of the year.