In fiction and in life there are often gatekeepers who guard the entrance to the castle or the enchanted kingdom. Consider St Peter, standing guard at the pearly gates, or those big blokes in dark glasses and wash ’n’ wear suits who stand outside the nightclubs and only let the pretty girls in. In both cases there are good reasons for them to be there: the big blokes are fulfilling a health and safety function by ensuring their nightclubs don't get overcrowded, and a public relations function by only allowing the prettiest admission (thereby establishing their club’s reputation for being a hot totty spot); while St Peter makes sure that heaven doesn't get filled up with non-believers and troublemakers, and therefore remains heavenly.
These gatekeepers do an unpopular but necessary job. So when people complain that literary agents are no more than self-appointed gatekeepers who are preventing writers from reaching editors they fail to consider what would be the result—to writers, publishers and readers—if agents stopped carrying out their literary gatekeeping role.
Editors are very overworked. A lot of their time is taken up by reading, and close reading at that. In order to do their job well they cannot skimp on this: editors were already horribly overstretched year ago; in the last year many have lost their jobs, and the books that they were responsible for have been handed over to the editors who remain employed, adding to their already too-heavy workload. This lack of time is nothing new: but it has been compounded recently, to a horrible degree.
No wonder, then, that editors prefer to work with agents. Doing so frees editors from the tyranny of the slush-pile; and they know that anything an agent submits is likely to be both publishable, and appropriate for their lists. It gives those editors time to work more closely with their writers, and to do their best to ensure that their books are the best that they can be. This means that we, as readers, have better books to read; and also that we, as writers, are displayed to our very best advantage.
There is a cost to the publisher: the contracts that agents negotiate are usually far more beneficial to the writers who sign them than a standard publisher’s contract, and so the publishers’ shares of income is cut: but the advantages of not having to deal with the mountain of slush outweigh this by a significant degree.