Fat tires can run lower pressure and the air cushion provides suspension. The main disadvantage of fatter tires is rolling resistance, which doesn't matter with an eBike.
The extra suspension is especially important on an eBike where the rider won't be pedaling as hard so will have more weight on the seat. There are basically three methods to address comfort on an eBike. Fat tires, suspension seat posts or a proper full suspension. Fatter tires are basically the cheapest solution. It's especially true of the eMopeds which are not designed to be pedaled (hint: If the seat height is fixed, it's not supposed to be pedaled). Those need suspension to preserve the rider's spine, and fat tires are a good way to accomplish that. Besides