I live in Arizona, where the water is very hard and very nasty.  When I moved into my new house, the first changes I made were to fix the water softener, and to install a Reverse Osmosis filter system for a tap at the kitchen sink and the refrigerator.  I bought this meter to see how well the filtration was working.  The tapwater is about 600ppm, and the water coming out of the RO unit is about 40ppm.  Having the meter allows me to check periodically for any deterioration of the RO unit.