Just to take the guess work out of it I think. For example... there's a natural spring by my house that I think should be pure enough to work, but I've been hesitant to use it because I'm not sure. Spending the $20 or so on the meter could potentially save me $2 per week on buying distilled water at the grocery store. Also, a lot of areas have local water sources that are more pure than others... some people may be able to use tap water (or tap water cut with distilled) if it's not too contaminated.But why is it needed??? If you know of a reliable water source already why you need a TDS Meter for???