Supervisor engines act as a "control" for all of the routing and switching operations (layer 2 and layer 3). It even affects how the individual line cards utilize the backplane of the chassis.
6500's aren't the only switches that utilize supervisor modules, 4500's do as well.
There are 2 types of device categories (from a hardware perspective) fixed and modular.
An example of a fixed device is a 3750 series switch, what you buy is what you get, if you buy a 24 port 100 meg 3750, you can't upgrade it to gig in the future, and you cant increase teh port density (although switch stacking helps with this limitation) without replacing the hardware completely, which increases down time, maintenance costs, and operational expenses. Most fixed devices have a single power supply with a DC input option, if a power supply fails, you have to replace the entire device, same thing for ports, ASICS, memeory etc.
On the other hand you have modular devices, such as a 6509 (routers can also come modular). This allows you greater flexibility in upgrade paths, including line cards, power supplies and even supervisor modules. The life expectancy of a 6509 chassis is much longer than a fixed device because of its modular approach, when you exceed port density, you simply add a line card without powering down the device, if a power supply fails, you simply order a new one, again with no down time, hardware failures are most often isolated to just the line card affected, so you would replace a single line card.
Fixed devices has the equivalent of a supervisor module built inside, which tells it how to use its hardware, but if it fails it costs downtime.
So you can see the benefits of a modular device over a fixed device is more service related than anything else. For critical applications, modular devices is the way to go, for non-critical applications, fixed devices can be a good initial costs savings, but other design factors come in to play such as expansion and end user activity (IE DB programmers require much more bandwidth than a youtube watcher).
HTH,
Craig
