what is a layer 3 switch for?

Talking to a sales guy today and the question came up asking what the point was.
without getting too complicated here is the answer as i see it:

1. switches are much faster than routers.
2. vlans and routers are conventionally used to break a network up in to segments so that problems in one area e.g. a sales floor dont affect another such as R&D
3. usually a separate router is used to route between vlans which is limited to the speed of the link e.g. 100Mb/Gb
4. all traffic between vlans has to travel to and from the router as well as a delay in routing calculations
5. this causes bottlenecks
6. layer 3 router is a switch that can do routing.  this means traffic can route between vlans at backplane speeds. So... bottleneck removed.

the extra bonus is usually you can get either a modular switch or stackable (cisco flexstack) which means you can plug e.g. a server into both switches providing high availability (ie everything keeps working if a switch fails).

oh and obviously by default a layer 3 switch is going to be manageable which means you can diagnose problems with individual ports

and ....

configure Quality of Service (QoS) which priorities voice traffic to prevent file downloads causing interference.