The justification is spending money more efficiently, which becomes even more important if you're a government entity and thus "your" money is actually other people's money gathered by taxation.

Put more bluntly, you cannot convince your bean counters to give you money you didn't spend, because if you didn't spend the money you didn't need it regardless your arguments for wanting/needing a seemingly extraneous budget.

Of course, this leads to the incentive of spending your entire budget no matter heaven or hell so you maintain your budget next year which can lead to actually wasting money rather than saving it.