I can offer some insight from the production standpoint.
Getting an agent and getting parts are two totally seperate things. You do not need an agent to get roles, however it certainly helps as agents have access to casting notices that are not released to the general public. Usually they are looking for a specific type for a role and the agents can submit their people. More than likely, the minor roles on a TV show (at least on the national drama law show I worked for) were cast in-house via database (i.e actors who caught the casting departments eye or were recurring (in different characters) actors or via open calls posted on Open-Casting or Central Casting.
You can get any role without an agent, you just have a better shot at getting an "in" with an agent and being considered a "serious actor." There is also the converse, I know several actors who basically book their own gigs and either rely on their agent to be "the heavy" and negotiate the fine print or just basically feel like paying someone 10% of their income for doing nothing. An actress I know and have worked with books her own gigs, negotiates the contract, arranges travel and got a role on HBO's Mildred Pierce and her agent basically does nothing other that show that shes a "represented actor."
Hope this helps.