PuShAkOv said:
But don't you only need it when you have engine problems?
A lot of the codes are for engine problems, BUT depending on the manufacturer there are a lot of other codes as well: Transmission, electrical system, fuel measurement and delivery, A/C, security, etc. Just about anything that is microprocessor controlled in the car is monitored.
There are a whole bunch of people out their who use scan tools to help tweak a few extra HP out of their car as well. It comes in handy if you are making mods (cold air intake, exhaust, etc.) and want to see the changes in performance.