I use a TDS meter when acclimating new fish. I have quite soft, low TDS tapwater. Often the fish have been kept in much higher TDS due to harder water or salt. I raise the quarantine tank tds & GH to avoid osmotic shock. It's easier on the fish to go from lower to higher than high to low.
I also use it to test between tanks before moving fish or before really big water changes. I use crushed coral to raise my KH but not an exact amount, tanks can differ.
I actually use the temp reading on my TDS meter every water change, faster & easier to read than a thermometer. Worth the <$20 shipped for me.