I've used a Tech2, actual GM. It allows you some stuff that you can't do otherwise. For example on the Bonneville I had there were a lot of things I saw while looking at the BCM. Things like illuminated entry/exit. When you turn the car off, the lights or such stay on for X time. Little things like that. You can also reprogram modules as mentioned
On the cars with a screen in the instrument panel like an Impala or 2000+ Bonneville you could change the start up message to say something different.