There are several states that can be accessed by first entering into a trance state.
Trance states are, essentially, transitional steps to bridging to things like the AP, Dreamstate, Voidstate, etc...
In a trace, what you're doing, ultimately, is severing your connection to the outside world via physical consciousness. This can be used to strip away all distraction and release yourself into the balance, to understand and experience an awareness.
It could also be used to, in the case of the void state, be used to sever your ties to time internally, this allows you to experience the veil, the void, and to a far lesser extent, also the dreamstate.
It's a preparation state before you can transcend to any higher plane or existence.