There’s a secret reason and wonderful result achieved when you practice the melody separately from the lyrics.
Some years ago, while I was helping an experienced singer learn a song for her next show, I noticed that certain spots in the song sounded off. Perplexed as to what was causing this, I began to troubleshoot. Instinctively, I asked her to sing the melody of the song without the lyrics. While she knew the words, we discovered she actually did not completely know the melody. As a result, there were sections of the song that sounded uncertain and imprecise.
I had her practice the melody using a single vowel sound like “ah” in place of the lyrics until she could sing it easily and with certainty. With that achieved, she sang the song with the lyrics, and the transformation in her level of confidence and vocal tone was stunning. I have used this technique with singers ever since, and it always produces remarkable results.
When we practice each element on its own, we strengthen control and knowledge of that aspect of the song so that by the time we make a whole cloth out of the song again, we’ve achieved a level of familiarity and control that far supersedes a haphazard learning process.