Assure maximum safety for flight crews (if any), ground crews, and equipment, while the vehicle is prepared for flight.
Avoid wearing out critical flight or ground equipment by activating it too long before launch.
Enable the launch director to launch at an exact instant - corresponding, say, to a favorable position of celestial objects, or to requirements for orbital rendezvous.
Synchronize launch preparations with supporting operations, such as the readying of radars and tracking cameras, or the setting up of road blocks near the launch site.
- Dr. Wernher von Braun
I think that for many hobby rocketeers with kids, that last bit may need to be changed to, "Synchronize launch preparations with supporting operations, such as the rounding up of the kids, the covering of the little ears, readying of radars, ...".