All on its own and isolated travelling through Kent, the M2 is just there to bypass some busy bits of the A2. Many people think it needs to be extended both ways to properly join London and Dover, or London and Margate, or indeed anywhere to anywhere else — but don't hold your breath.
Massive works to upgrade the M2 finished recently. Before, the A2 at its western end was three lanes wide, while the M2 had only two. The works upgraded the M2 to four lanes for part of its length, and brought some huge junction improvements, since some junctions were worthy of the lowly M50. This actually involved building a second Medway Bridge.
The original plan for the M2 was of course to get traffic from London to the Channel Ports, but this never happened. It is unlikely to be extended now that the M20 does that job.
With thanks to Simon Hollett and David Biagi for information in this section.