> Are electric bicycles desirable mainly because of lower physical effort, or higher top speed?

I don't have one myself, but I know that in Europe, they can't go over 25km/h (higher than that, they're not considered as bicycles anymore) so the interest is to lower physical effort.

> So why would I choose an electric bicycle over my traditional one?

Not everybody enjoy exercising on their way to work, but even if they do, with electric assistance, they can cycle on longer/hillier commutes than what would be possible/convenient with a regular bike.