I'm not an expert in this by any means, but let's say you're a technical diver (or you want to know your actual SAC or RMV), by over or underestimating your tank capacity, you may be under or over estimating what is actually happening.
As a rec diver, this is probably not a life threatening issue, they probably aren't betting their life on SAC/RMV, but who wants to gamble on close enough if you do have to make those calculations?
