Based on the usage I've seen in West London out to Heathrow I've the following observations.
People are using them instead of cars, bicycles, walking or public transport for the following reasons.
No parking issues at home or work. Can avoid the close contact with others, unlike public transport.
No tax or insurance, which is very expensive in London; especially if you are young or have a poor driving record.
The can access the same route as a pedestrian, but 5 to 6 times faster with near zero fatigue.
The short order chef at our local travels 8 miles each way to work from Egham to Hatton Cross at a speed not far under that of a car.
The route is 90% the same as the car, but is considerably faster through Staines. Given that the shortest route has a 20mph speed limit the scooter is actually faster. There is no direct public transport option, without many changes.
At first I thought it was mums and house husband's riding little Johnny's scooter home. I now see all ages using them around here.
It has increased massively this year and I don't mean youth or child use.
Some might say why not use a bicycle, however you have a storage/ security issue at both ends of your journey.
A scooter just follows you everywhere, tucks under your desk at work, getting charged with the companies electric.