When a preconceived solution goes in search of a problem, there is often a detour via the Exchequer.
When the solution is a mammoth construction project, say an underground railway through a city, the detour can be horrendously expensive. Far better to identify the problem first, make sure it outranks the many others seeking public funds, and then try to find the most cost-effective solution, without preconceptions. This businesslike procedure is actually mandated by the Public Spending Code to which all Government departments and agencies are nominally committed. The intention is to avoid waste in the public capital programme.