Plex is designed to fill a need that is left wanting by the existing Python regular expression modules. If you've ever tried to use one of them for implementing a scanner, you will have found that they're not really suited to the task. You can define a bunch of regular expressions which match your tokens all right, but you can only match one of them at a time against your input. To match all of them at once, you have to join them all together into one big r.e., but then you've got no easy way to tell which one matched. This is the problem that Plex is designed to solve.
Another advantage of Plex is that it compiles all of the regular expressions into a single DFA. Once that's done, the input can be processed in a time proportional to the number of characters to be scanned, and independent of the number or complexity of the regular expressions. Python's existing regular expression matchers do not have this property.