And there's always the exact right key for any performer's voice. It can be a matter of a half-step higher or lower for the song to come alive for any particular singer. Beginning guitar players often get stuck singing in a key they are comfortable playing the guitar in, instead of a key that's exactly right for their particular vocal cords. I think we usually need a short length of time to get prepared to listen to a song. The introduction of a song provides that time, sets the mood, and therefore can be a crucial element to the success of any arrangement.
Any song is an adventure.
As a performer gets to know a song and sings it over and over, a sense of how to begin it can evolve. The style of the intro will depend on the song, and at best, it will draw you in, catch your attention in a compelling way. As the song moves along, there comes a point where enough has been said and a musical idea is needed that deepens the adventure and sets the song up for completion, or payoff. And again, the style of the song and its story or quest can imply what sort of rest, expansion, or perk the break should deliver. The ending of a song has a different function from that of the intro, needless to say, but it's not just an ending.