The intro tone is very important if you want slick overlays in auto from DPS.
Strictly speaking, the intro time is the time from the start of the song (or the cuein) until the vocal starts.
If there is a very long musical passage before any vocal content on a song, some people set the intro to a major musical break or drum-roll, etc.
The intro denotes at which point a tracked or dry overlay will stop.
If you don't bother to set any intros, then your automation will not be as slick as you would like!