Agent representation

The agents are used to find and hire the most suitable people to write for certain television companies - that are best suited to certain types of genres, styles and times etc.
The agents debate contracts with the television companies and how much the writer and themselves (the agent) should be paid.

For example if the BBC wanted someone to write a comedy series aimed for a older audience they would probably look for people with previous successful experience in that area of expertise. writers that they would possibly consider for this type of drama would be; John sulivain (late writer of only fools and horses) or John cleese (writer of Faulty towers) etc.

In my opinion I think agents are not really that vital in certain circumstances for instance the idea above about writing a comedy drama for a older audience - you'd use older previous writers and you wouldn't really need knowledge of a agent to find a capable writer for that.
But on the other hand when trying to find a new fresh writer for a totally new and original drama you'd need to find someone different and unique to any other shows and therefore need the expertise of a professional agent.