It tells you how much your plant took up and how much spits out .od.

Runoff can be decieving depending on if there are "channels" inside the medium which are just areas that is easy for water to pass thru... I have seen buckets with dry spots inside desite being heavily watered. Depending on your medium and nutrients in it, the initial lush from the runoff could be much higher than what you actually have since osmotic nutes will become concentrated and have higher levels initially. Plants require a surprisingly small amount of nutrients and won't take up more than they can handle if given a choice.
In a hydro setup, it would be much more accurate to check inflow VS outflow, but with soil or similar, a lot of nutes and water get hung up in the medium, making it hard to get an accurate idea. Ph and buffering additions can change the count by a lot as well. My philosophy was to worry less about the numbers and try to keep the plants happy. Use the Force, Luke!

if you want to learn about your soil you need to do a slurry test. if you want to know the numbers in out as fo PPM and etc then use the runoff.