There is a Passuk (verse) in Parshat Ki Tetzei that reads “And if you desist from vowing, no sin (fault) will be found with you.” This implies (and confirmed in a Gemara in Nedarim) that one that does vow will be found at fault, even if he/she fulfills the vow. Why is this true? What if someone vows to do a good deed, what could possibly be wrong with doing that?
Jonny Gewirtz in his weekly publication Migdal Ohr offers an insightful answer: Since one could have fulfilled the mitzvah without the vow, the vow merely serves as a potential obstacle because if they do not fulfill the act they have committed a sin by transgressing their vow. On a deeper level, though, one who desists from making vows will not be found sinning because they are aware of the power of the tongue. They know that speech, once uttered, cannot be retracted, and thus is careful about what they say. This awareness applies not only to vows but lashon harah, hurtful words, falsehood, etc. which encompass so many other sins they will be able to avoid.
At the culmination of Elul on Erev Rosh HaShana, and again at Kol Nidrei on Yom Kippur, we annul any vows we have taken and declare our intention not to vow again. This is the hope of the new year, that it will be one in which we will be cognizant of the power we have in our tongues and in our actions, and speak/act appropriately. This undertaking to be careful with vows is not the ultimate goal, it is just the beginning.
