Keeping the grid stable
A dead battery in an EV leaves the user uniquely stranded when compared to internal combustion vehicles, so the ability to charge an EV from any AC supply is important. Batteries use DC, meaning somewhere between the mains and the battery a conversion needs to be made. This slow-charge ability requires the EV to be able to rectify AC on board.
Ideally, each EV would be identical and would charge the same. In reality, however, minute flaws and manufacturing variables in the rectifying components create harmonics in the mains AC signal. Multiplied by hundreds of thousands of EVs, all introducing their own perturbations, and the frequency profile of the grid quickly becomes a noisy mess. This noise represents an obvious waste of electrical energy. Conductors carrying high frequency AC also exhibit far higher resistance than those carrying the same power at lower frequencies, further reducing efficiency. But more importantly, a noisy AC signal causes interference in high frequency applications, which can negatively impact performance.