Because we live in a democracy, and in such a system, those who pay the bills (the taxpayers) have a right to know how and why their money is being spent.
Moreover, it is surely much better that the taxpayers and voters know that decisions costing billions of pounds have not been arrived at lightly or casually, since otherwise, defence spending will become a lower priority in the public mind. The services who explain their requirements and procurement priorities most clearly will be those who gain greatest public support, and politicians will find it easier and more popular to spend money on those programmes.
The RAF have not done a good job in this area, and as a result, its priority programmes probably enjoy less public support than (say) the Navy's carrier programme, or the Army's Special Forces, and even it's battle to save particular 'cap badges'.