I well remember someone I knew well who had an MoD procurement budget. Every late Feb/early March he'd be looking around for things to buy, so he could use all his budget. He knew only too well that any underspend would result in a lower figure for the following FY.
The MoD spends money for lots of reasons, and sometimes they happen to get something the sailors/soldiers/airmen actually need.
I expect that the person who bought the Mk3 Chinooks thought he knew a lot about the requirement, too, don't you? And then there's FRES, and then there's..