Q2A fires events. Events are usually (but not necessarily) actions performed by users. Sample events are voting on questions, selecting answers, user registrations, etc. Those events are volatile, I mean, they are fired and then get lost, although their effect remains, of course.
The event logger is a simple plugin that logs all those events so that they don't get lost. So if you take a look at the logged events in the log you will see some information (called parameters) about the events. Each parameter depend on each type of event (votes, registration, etc).
So there is nothing special about this plugin. However, many other plugins use its output to reconstruct the history of the site. For instance, you could see what user voted which question and when. This, of course, is not reliable as the event logger can be disabled and then reenabled so all events fired in the meantime would be lost. So the plugin in its own is not special but it might be a dependency for other plugins.
One important drawback to consider about the event logger is that logging every event in the site generates an additional processing that, in most cases, is not required.
