Bruce Bartlett offers a brief history of the budget:
Ironically, the portion of the budget over which Congress actually has meaningful control has fallen sharply over time. In 1970 the discretionary portion of the budget--those programs and operations subject to annual appropriations, was 61.5% of all spending. The rest consists of mandatory programs like Social Security, Medicare and interest on the debt that are not subject to annual appropriations. Their spending is automatic and cannot be reduced just by appropriating less money to them. In 2009 the discretionary portion of the budget was down to just 35.2% of spending.
Given all these changes in the budget process, the president's budget has been greatly diminished in importance. Whereas it was once the necessary starting point for all budget discussion, since that was the only place the numbers even existed, now it is just one proposal among many. Congress tends to rely exclusively on the CBO for all its budget numbers and analysis. Although departments and agencies are supposed to adhere to the president's priorities, they do so only half heartedly.