It's not always convenient (or even possible) for your everyday transportation to also be exercise. Electric assist extends biking range (and makes biking possible) for people who would otherwise need to use another mode of transport -- because you have an important meeting today and don't want to be sweaty, because you are sore from weekend exercise, because the office is a bit far, because you have to pick up groceries, because you're 70 years old, because you're 8 months pregnant, because you have a cargo bike, because you have a serious bike race later in the day....

Electric assist is fairly common in the Netherlands (I have family there), where a pretty big percentage of the population uses bikes as the basic mode of transport. It's really sensible to have the option to ride somewhere without it being a workout.