They are much more relevant if you have a marine aquarium.
Most people test for TDS when they're adding freshwater/RO/top-off water into a salt water aquarium (to replace the water that has evaporated) or when mixing new salt water, in which case the lowest TDS possible is desired. Testing the TDS of your aquarium be it marine or freshwater isn't really anything to go by as much as other parameters are. There likely are a few niche purposes for Freshwater aquariums, but as a whole I think it's something most people don't need to bother with.
I have about 65 TDS on average, straight from the tap.