The quality of the thing offered decides whether the offering works at all. A blemished animal, the sick or lame beast a man was going to lose anyway, does not count, because sacrificing the leftovers is not sacrifice. The old distinction between Cain and Abel is here written into law. Bring the best, unspoiled and whole, or the transaction with reality simply fails.
A vow is a bet placed against your own future conduct, and you cannot settle it with something defective. The demand for the unblemished beast is a demand that you take the act seriously enough to give up what you would most want to keep. Offer the flawed thing and you fool no one but yourself. The offering has to cost you. What you lay on the altar moves nothing above you unless it hurts to give.