It means you can tax car usage more heavily than would be the case if you just taxed their electricity consumption at the same rate as electricity used for other purposes (like home lighting and heating), in a way analogous to how in the UK petrol and diesel used by cars is taxed much more heavily than petrol used for other purposes (eg heating, or agricultural vehicles). This is useful because it discourages heavy use of private vehicles and especially of inefficient ones (compare the UK or Europe to US car usage, culture, and vehicle preferences -- I can't help thinking US cheap gasoline is relevant there).

In the UK, petrol is taxed much more heavily than electricity currently is, incidentally.

But mostly this is about the fact that currently petrol and diesel taxes bring in a lot of money, so if we don't shift that onto "extra taxes on electric vehicles" then there's going to have to be a rise in general taxation -- which is generally not popular.