Is your house plumbed in copper or plastic?

The dielectric fittings are there to prevent galvanic corrosion between dissimilar metals. If your house is plumbed in copper pipe you don't want it in direct contact with the steel of the hot water heater. So the fitting electrically insulates the copper from the steel.

If your house is plumbed in plastic then dielectric fittings won't make any difference either way.