I consider the Stoney Point OAL as basic equipment for reloading just like a set of calipers. Unless you are reloading strictly domestically produced, modern firearms that you can be pretty sure have throat dimesions that coorespond to SAMMI specs, using this tool or one like it, is the only way to accurately measure the distance to the rifling with various bullets. One thing I had found is that using a dowel rod through the bore to push back against the tool pushing the bullet from the chamber is escential for a prescision read of where the rifling begins.
The bullet comparator is necessary to use this information (the distance to the rifling) if you are really interested in seating the bullet consistantly since the length of the bullet to the tip varies, especially with soft nosed varieties. The comparator allows you to measure on the ogive.