It doesn't do anything you can't do with a pencil and paper. It just makes things quicker and easier. Download the trial of BeerSmith and check it out. I really like it, but it's the only one I've used other than BrewPal on my iPhone.
BeerSmith has all sorts of tools built in for conversion, temperature calculators, water calculators, water profiling, equipment considerations and corrections, and it stores your recipes and results.