Chains are reliable, easily replaceable and inexpensive - but they do require frequent cleaning and oiling to run smoothly, which can be messy. They wear out and need to be replaced after a few thousand miles on a normal bike. Not only do the chains wear, but the gears that the chain meshes with also slowly wear out. So when you replace a worn chain, the sprockets and chainring usually need to also be replaced at the same time or the chain will skip. Electric bikes, with their powerful motors, exert more stress on a chain, which can lead to even more wear on a chain, and in extreme cases, chain breaks.
Automobiles have mostly moved away from external chains because of the lubrication problem - belts are used outside the engine where there is no automatic lubrication, (like for air conditioning and the alternator) because of their long service intervals and virtually no maintenance. Made out of polyurethane, carbon belt 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 Rohloff or Nuvinci), belt drives have produced a super low-maintenance commuter bike.
While the advantages are many, there are limitations. For starters they are a heavier, premium upgrade and will only work on bikes with internally-geared bikes with a "split" frame .A belt drive requires an opening in the frame, often positioned where the chainstay and seatstays meet at the chain-side dropout. Since you cannot take apart a belt like you would a chain to resize it, installation is a bit trickier and you will need the right size of belt for your frame. Rear wheel removal to fix a flat is much more of a challenge with a belt - it needs to be re-tensioned which can be tricky if you have never done it before. Some of the internally geared hubs have a more limited gear ratio - the ratio of the highest (for speed) to the lowest gear (for climbing). Derailleur gears are typically 300-400% with a single chainring, 4-500% with two chainrings. Nexus 8-speed has 309% which can be limiting if you want to have speed and low gears for climbing. Rolhoff 14 speed hub has a gear ration of 500%. Nuvinci 380% range and the Nexus 11-speed 411%.
All things considered it is a great option if you don't want any worries about maintenance or greasy pants, or just like the clean look.