Some e-bikes operate in pedal-assist only, others have a throttle, and some have both. Generally, pedal-assist only bikes will provide multiple power settings to choose from to help customize your ride, while bikes with both throttle and pedal-assist will have limited pedal-assist options. With these bikes, the throttle provides full control (when needed) while pedal assist is just a secondary option, great on straightaways or open road.
There are individuals who claim to have lost considerable amounts of weight by using an electric bike. A recent prospective cohort study however found that people using e-bikes have a higher BMI. By making the biking terrain less of an issue, people who wouldn't otherwise consider biking can use the electric assistance when needed and otherwise pedal as they are able. This means people of lower fitness levels or who haven't cycled in many years can start enjoying the many health benefits E-bikes have to offer.