I use my SAC rate for doing vintage dives when I use no SPG. The SAC is the only way to get an idea of how much air I will need to do a dive for a certain time at a certain depth.
I also use a J valve in combination but usually I'm already back close to shore about ready to finish the dive when I need to pull the J.
Instead of looking at my pressure gauge I look at my watch and depth gauge.
How do you know if your SAC is still the same as the SAC you planned on when you were on the surface (it could have changed due to unpredicted/expected increase in exertion level U/W during the dive)?