The money is committed and is taken from the relevant budget, so the budget holder knows how much they have left to spend.
If the money wasn't committed the budget holder might forget they'd ordered �x worth of goods and might order something else as well which they couldn't really afford.
A budget print-out will show them what they have ordered and so they won't over order any particular item. They will also have the names of the suppliers on record if they want to order from them again.