Tap water contains extra ions (ie not jusy H2O) which conduct. Deionised (as the name suggests) has no extra ions so it is just H2O and which does not conduct as it is not ionically bonded.
Low conductivity is better if you don't want electricity to pass. High conductivity is better if you do want electricity to pass. Also, water itself is actually an insulator (very low conductivity)...
Electrical condunctivity in water? Either there's a very small amount of water, or a very large amount of something that doesn't conduct electricity in the water sample. Like dirt. Mud will not...
conductivity is not zero because water itself has ions i.e. H+ & OH- so conductivity cannot be zero. even if you use EDI still some conductivity will be left it will be almost negligible but...