A melody, is the heart of a song, a composition, a masterpiece. Without melody, a song is lifeless, like an empty vase. Like love at first sight, you can instantly fall in love with a song just by listening to it's melody without knowing the lyrics.
A lot of singers (especially young ones) I know loves to improvise every melody of every song he/she sings, until the original melody is completely gone. So, what's the point of singing that tune then? I mean giving some embellishments here and there or improvising making a tune sounds more interesting is fine, but when someone improvise a melody from the beginning to the end, it will only make a song pointless. It's like getting a brand new BMW sports car and pimping it to the max until it looks like a parade car.
For you young singers out there, think about it, the real challenge here is not to sing a melody with fancy technique or with a lot of added notes and crazy high notes whatever. The real challenge is to be able to sing a simple song, and touch people's hearts. When you improvise a melody too much, it sounds more like you're trying to compensate a bad singing :( it sounds rude, but, really. Check out Frank Sinatra, as a pop singer of his time, he didn't need too much of improvising to become world famous and loved by people all around the world, he just sings the original melody, and give it his own original touch, simple and strong.