It's a lazy way to budget. Agencies and subordinate orgs should be required to put forth a estimated budget each year, with the leadership of each organizational tier required to audit and approve these requests. There should also be an incentive structure to reward accurate forecasting and punish budgetary gluttonary (ie, "just give me all the money you can" and then wind up not needing it).
But all of that requires a higher level of accountability, a higher level of care and scrutiny, and ultimately, a higher level of leadership at all levels.
So we get left with the current "use it or lose it" system. That results in a LOT of waste. When I was at Dyess AFB, we had an entire storage closet FULL of flat-screen TVs still in boxes, completely unused...bought with "year-end money." At Little Rock AFB, one year our commander spent thousands of year-end unused funds on artwork for the squadron walls. I mean, the artwork was cool and all, but was it really necessary? The answer to that is "no."