I once heard a classically-trained musician say that musicians are really just small-muscle atheletes. Whether we are musicians or atheletes, or both, one's voice is at least as important as one's guitar or one's legs, so it follows that it demands and deserves proper training.
One can teach one's self to sing, or to sing better, but that's true of any activity. The reason we pay an expert to help us along that path is because said expert is either better at it than we are, or because we recognize that we can progress faster, or both.
Spent wisely, voice lessons, nay, any and all lessons, is money well invested.