Wayne,
When you dyno a freshly built motor, you can:
Break it in in a controlled environment,
Check for leaks very easily,
Tune it (with verification) for the most power your combo will allow,
Get the exact HP and TQ figures for your motor,
If you chassis dyno it later, you can figure drivetrain loss.
It's not entirely necessary for the street, but after spending $X,000 on a new motor, what's another $500 bucks to get the most out of it?
Michael