Because mechanism designers do not generally know which outcomes are optimal in advance, they have to proceed more indirectly than simply prescribing outcomes by fiat; in particular, the mechanisms designed must generate the information needed as they are executed.
The theory of mechanism design can be thought of as the 'engineering' side of economic theory.
Devising a mechanism is a lot like solving a puzzle - and gives you the same kind of kick.
Leo Hurwicz is the father of mechanism design theory and has inspired much of my work, and Roger Myerson is an old friend and collaborator and a tremendous economist.
If budget planning requires gathering information from people who may not always have the incentive to disclose that information, then the principles of mechanism design can definitely be of use in such planning.