Other than reporting error codes, is a scan tool useful? Can it help you to tune the car to optimize gas mileage for example or other helpful things?
When it comes down to it, a scan tool is irreplaceable. It used to be you could short two pins on the DLC and make codes flash on your dash. Now, you get a light on your dash and you need a way to figure out what it is ... that is, if you don't want to fully trust your mechanic. The right scan tools can also be used to diagnose anti-lock brake issues or supplemental restraint systems (air bags). Without the tool, you would either be replacing parts blind or are at the mercy of your mechanic.
As an aside, you can purchase apps for your iPhone or Android devices which can allow you to tap into the codes and information through a bluetooth connection device. These can be had a reasonable price for those who don't want to spend a lot of money a hand held device with fairly good results. I would bet, someone who is truly of an entrepreneurial spirit could take one of these bluetooth gadgets and write some code to make it do all sorts of things. With OBDII and the CAN Bus, the sky is nearly the limit.
The answer is yes, with there being specifics for your car, of course. A good scan tool will tell you the instantaneous fuel consumption (which many cars have as a part of the instrument cluster now). You can view the exact coolant temperature being measured, which can be helpful for troubleshooting the cooling system. On some vehicles, with the right scan system, which may even be proprietary, you can change settings in the vehicle computer. You can change performance settings this way, although I think most cars nowadays are already optimized for the best gas mileage. Information about the ignition timing is available this way, too, and that can be helpful for troubleshooting.
I definitely agree with @Paulster2! I would like to add this though, bluetooth scan tools do exist as well as some limited Wifi ones. OTC makes the Encore. However though, you need to keep in mind what kind of scan tool you want. There are the ones that you can buy from Autozone that cost $100 - $200 and there are professional scan tools that range from $2500 - $7000.
Personally I have a range of them. I tend to use certain ones for certain applications as well. I generally use OBDLink bluetooth scanner for my Miata since it's a bit older and doesn't have the data bus systems in it. I use my Snap-on Verus for mostly Asian and Domestic brands, and I use AutoEnginuity for mostly Chrysler and European brands. They all have their ups and downs. It just depends on what you want or need. I used to work in a dealership and owned my own shop for a while, so advanced scan tools were literally necessary. If you're a DIY'er you wouldn't need anything too expensive. I would say either OBDLink or AutoEnginuity would suit your needs perfectly.
If you're wanting to tune, that requires an OEM or factory scan tool. At Honda we had scan tools specifically made by the guys at Honda. Same for Chrysler, they use "StarScan". SOME scan tools like the Verus can do this, but it's generally unlikely you can with an aftermarket scan tool. There are replacement ECM's you can buy to do this though, they generally come with the software and everything. If thats what you're wanting to do of-course. Don't get scammed by Ebay fuel tuners either. There is an episode of a show called "Mighty Car Mods" on YouTube and they literally debunk the myth of plug and play gas savers with a guy that engineers engine systems.
Yes in short, you can do almost anything you want with the right scan tool. AutoEnginuity is the cheapest most versatile scan tool I've ever come across.
Good luck with finding the right one!