Metrics should serve the needs of society ... but the easy metric that has been most important in the Breton Woods institutions and their development clones has been disbursement. The World Bank and the others have used disbursement as a metric for performance for decades even though this measure on relates to progress and performance in a very indirect way.
There are cases where disbursement is a useful proxy for results, this is not so when it is used to the exclusion of almost anything else.
Disbursement serves as a fairly good proxy for activity ... but does not measure results other than in a very controlled and stable environment. By definition, development, when it is successful, is not stable, but progressing ... and the only metric worth having is are measures for progress and performance.