Traditional derailleur external gear sprockets and chains are the standard for most bikes, including eBikes, for good reason - they have great gear range, with lots of torque, easy access to service and change flats. Chains can get messy though - and then there is the service life - a few thousand miles - and the service interval - they need to be cleaned and oiled every few hundred miles. Internally geared hubs allow the rider to easily go from a harder gear used at higher speeds to an easier one as the rider encounters a stop sign or light without having to plan ahead,
Lower maintenance (aka long service interval) is another sought after feature of the internally geared hub - less exposure to the elements mean less wear - but when service is needed it is more technically challenging than a derailleur bike. Internally geared hubs allow for the use of enclosed chain guard - or better yet a smooth, clean, quiet and low maintenance belt drive. Of course there is no free lunch - internal gears are a premium upgrade, they are heavier, and may reduce the max torque available from your motor.
The norm for most bikes, with great ratio range, lots of low speed torque as well as top end gears for high speeds, and easy access to service and change flats. For eBikes shifting over the entire range when the bike is at rest can be a problem, a challenge less active cyclists face at first. They can get your trousers dirty if you aren't careful. Good quality gears can last with frequent maintenance, but will wear out faster than internal gears and belt drives.
Internally geared hubs are standard on Euro commuters - they are popular with less experienced cyclists because internal gears can shift over the entire range at rest (useful if you forget to downshift after coming to a stop) without causing problems, they tend to be quieter, cleaner and less maintenance than derailleur gears. Flat changes are more complicated, but overall maintenance is needed less frequently, and if used with a chain guard can keep your trousers from getting greasy.
Carbon belt are a premium upgrade for a super long service interval (among other benefits) work only on bikes with internally-geared bikes with a "split" frame. Carbon drives don’t require lube, making them a cleaner option. They are also lighter, quieter and more durable than a chain. Service life for a belt is at least 3 times that of a chain. Paired with today’s wide-range internally geared hubs (like a Rolhoff or Nuvinci), belt drives have produced a super low-maintenance commuter bike.
The NuNinci Continuously Variable Transmission (CVT) internally geared hub has a unique abilities to eliminate gear steps. So you don't have to think about changing into the right gear, just twist the grip until you reach your preferred rate of pedaling (cadence).
Rolhoff is really the top of the line gear system you can choose - use with either a chain drive or belt drive - with an astounding 500% gear ratio and 14 different gears. Rohloff gears are sealed against the elements in an aluminium hub shell, so maintenance is minimal, a minimum of service life 60,000 miles and a minor service interval of 3000 miles. It is also heavier, but hey you are on an eBike!