RO water has a TDS of zero. The distilled water I buy at my grocery store has a very low TDS. Most brewers would modify RO and distilled water for a target ppm (parts per million) of Ca Mg HCO3 SO4 Na and Cl. The TDS will literally tell you how many ppm of these you have in the water. You cannot isolate individual components of say Ca vs Mg but it will give you the total of all of the above as a reading. This could improve your consistency for example.
Also if you are using municipal tap water a TDS meter could help you monitor any changes that might be happening there as well. For example my tap water today is 42 ppm of TDS.
TDS is important for one of my other hobbies, breeding fish and shrimp. You have to monitor depending on what you want to breed.