You enter a line and price, it tells you what half points and onward are actually worth in terms of actual games played.
Or you can use it in reverse and enter a line and then enter the next point (eg "buy the point") at -120 or whatever your book is charging you. From there, it will tell you the lost or gained edge by paying for this point. Usually buying points results in changing from -4.5% EV to even worse, but there are a few cases where buying points is actually helpful and can reduce the edge from -4.5% to, say, -3%.
Um, err, sort of, but not really the intended purpose
The raw information it is based on is the value (percent probability) earned from point to point. So I suppose if you are trying to compare your price and, say, a closing line that is a different line, yah you could use it... but not really the appropriate tool for that. Essentially it helps convert from spread and total to a percent (although you can also use a push chart, which is all this really is under the covers).
For calculating your own edge, just compare your line to the market line.... what else do you need???
Having said all this, the half point calculator is by far the #1 most useful tool in the SBR tool menu, and every person here should learn what it means and how to use it.