Publisher / Agent

In the course of my research I have found that many publishers, especially the very large ones, do not take “unsolicited” manuscripts.

I presume by this they mean one must have an agent.

Yet there are also some publishers who will take manuscripts direct from a writer. In the case of the latter does the publisher also act as the agent ie: marketing the book, looking for ways to attract attention, looking for new markets for the book, acting as deal broker between author and publishing overseas/translations etc? In other words, how much does an agent differ from a publisher who takes manuscripts and publishes directly?