Well, it will let you know if what you're doing is helping or not. I tend to leave some lower branches that don't get much light just to do tests in flower. You can also take fan leaves and get sap from those.
Spraying in flower boosts your levels. You can see the difference after spraying Aloe you can also measure it with your meter. Or, if something actually hurt you can also measure with the meter. It's a tool to use to tell you if you're doing things right or wrong.
The brix scale was invented a long time ago by a scientist attempting to help grape farmers grow the sweetest grapes quite some time ago. He discovered that you could measure the change in light refraction relative to sugar level. He invented the scale, it is arbitrary.