From my perspective, nothing 'inside' or official, but just like any industry...

If you're a newb, you need to establish yourself, gain contacts, etc. An agent is a good start.

Once you're established, like any other job, you have connections, you may be able to handle your bookings on your own.

However, if you're too big, you might need the assistance of someone else to handle your bookings - like an agent (back to square one), manager, suitcase pimp, etc.