Sahih Muslim Book 26, Hadith 4
The vow neither hastens anything nor defers anything, but is the means whereby (something) is extracted from the miserly person.
Allah's Messenger (may peace he upon him) singled out one day forbidding us to take vows and said: It would not avert anything; it is by which something is extracted from the miserly person.
It does not (necessarily) bring good (in the form of substantial, and tangible results), but it is the meant whereby something is extracted from the miserly persons.