It's also very useful for finding places. If you know one place that's been reported, but you can't figure out the next one, because of bad writing, spelling, etc., you can use the voyage tool to plot their voyage from the first place to the second one, and you can see where they end up. Then it's fairly easy to figure out just what place they're at.
Sometimes they don't correct for the magnetic deviation. One time I was using the voyage tool on Patterson. She was coming down the coast of BC from Alaska, and I couldn't read one place in their log (well more than one, but you know..). When I plotted their voyage, the map showed them sailing across the Coast Range. Not even the Patterson could do that. When I entered the magnetic declination for the year and location, the voyage showed properly.
You can get the magnetic declinations from the Historical Declination Viewer here