What is the purpose of chairmen at conferences?
Depending on how the conference is organized, the duties of a chair before the session can include:


Selecting the abstracts that will be presented within a given session.
Organizing the abstracts in the order of presentation.
Transferring unused abstracts to other sessions (where possible).
Advertising to solicit contributions.


During the session, the goals of the chair can include:


To introduce the speaker
To ensure time constraints are being adhered to
To moderate a question-and-answer session following the talk
If other questions have not been asked, to offer questions of her own.


The organizer of a session may or may not be the chair of the session. In larger conferences, in which you have many parallel sessions (some have 50 to 60 or more run simultaneously), it is entirely impractical to have a meeting organizer chair every session. For smaller conferences, however, this is done. In such cases, though, the organizer of the session is still called the "chair" of the session. 

It is also possible, at some conferences, that the organizer is unable to attend the session, as a result of illness or conflicts, for instance. In such cases, an "emergency" chair is appointed to run the session. (I had to serve in such a capacity at the most recent conference I attended.)