Most professonal grade drafting tables have a wooden bar that is
spaced about 1/4" from the desk top, and runs almost the length of the
desk. This bar faces the person sitting at the desk. I was just
wondering what this bar is designed to be used for.

The bar you're referring to serves a useful function when a draftsman
is working on a large drawing, and wants to bring the top of the
drawing downward on the table for easier access. When the draftsman
scoots the drawing toward himself (so that the upper section is now at
the bottom of the table,) the drawing slips through the "slot" between
the bar and the table, and the draftsman can "belly up" to the table
without his body smearing the drawing, since the bar serves as a
barrier.

You mostly see this kind of bar on older drafting tables. Many more
recent tables use a device called a Spiroll Drawing Protector, which
allows the unused portion of the drawing to curl into a protective
tube.

Engineer Supply: Spiroll Drawing Protector
http://www.engineersupply.com/catalog/10Expand.asp?ProductCode=ES71

My Google search strategy:

"drafting" + "drawing protector" 
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=drafting+%22drawing+protector

I hope this information is useful. If anything I have said is unclear,
or if the bar I've described is not the one you're thinking of, please
request clarification before rating my answer, and I'll gladly offer
further assistance.

Best wishes,
pinkfreud