I really haven't been using one yet, but for soil you would normally use it to check the nutrients going in and maybe the runoff coming out of the pot. That'll give you an accurate measure of the PPM and PH inside the pot. Water PH is important, but organic nutes usually have a buffer included that stabilizes PH in soil.
PH may be more important than PPM in soil. You'll be going by the directions on the label for your nutes and normally mix it by the gallon, so you should know going in you're okay with that.
I'll let others chime in that do monitor on a regular basis. I'm about to have a couple of Octopots and hybrid type planters that use soil and a reservoir for water/nutes andI'll have to be on it more then.
Best vibes! peace