A low step frame (a.k.a. wave) is a style of frame often used for an upright comfortable riding eBIke. Traditionally (but no longer), low step and step through frames were called "Ladies Bikes", mainly for the advantage wearing long skirts or dresses - however these days low step frames are popular with all genders, ages and abilities. Materials technology back in the day was not what it is today, and the low step frame was not as strong or stiff as it can be made now. On eBikes, any weight penalty can easily be paid for by the motor and battery.
Easy entry to mount the bike
Comfortable riding frame bicycle frames that flex vertically (but not laterally) can add comfort to the ride, similar to suspension.
Quick on/off Great for frequent getting on and off
Less risk of ripping or stretching clothing
Alter-abled For those with reduced agility (eg knee, hip or back challenges), swinging a leg over can be a literal pain, or might cause re-injury.
Heavier. Compared to a traditional diamond frame consisting of two near-triangles, open or step-through frame designs must be designed with thicker gauge tubing, the use of additional gusseting members, and/or monocoque frame construction.
Accessories Fewer places to mount a lock, air pump or water-bottle.
Rear rack mounted battery - Integrating the battery in the frame is very challenging on a low step bike because it is a single tube with cutaway sections to allow the battery to be inserted, rather than a complete tube that would provide enough strength and stiffness to be low step. The battery in back can make the bike somewhat top heavy, and the weight in the rear makes handling feel a bit like the bike is swimming side to side. However, for the comfort city rider who is not speeding around, this effect is not very pronounced.