It's a solution to a problem found by many new cyclists.that's a fashion statement, a bit more than extreme.
They feel unsafe on a traditional geometry bike because with the saddle set at the correct height, they cannot plonk their feet firmly on the ground.
You see a lot of ladies riding with low saddles for this reason.
It will also help if you have limited mobility and find popping forward off the saddle to stop difficult, and inevitably, find it equally hard to start by pushing on the pedals while astride the bike and then popping up onto the saddle.
Gazelle call the bike Balance for a reason.
Most of us manage fine on a traditional bike, but I would find a Balance easier if I was doing a lot of stopping and starting which might be the case in a city with lots of traffic and junctions.