People who come to me from a yoga background usually spend the first class quietly trying to work out which bit is the exercise. I watch it happen.
Where it comes from
Qigong is Chinese and considerably older than most of what gets taught as yoga in the West. It is not a branch of yoga, a warm-up for it, or a gentler alternative to it. The two traditions grew up thousands of miles apart and are answering different questions.
The reason they look related is that both spent centuries paying attention to breath and posture, and there are only so many ways a human body can stand.
What is actually happening
Repetition, mostly. A movement is done many times, slowly, in a way that stops being about the shape fairly quickly. In yoga you often work towards a position and then hold it. Here the movement itself is the thing, and there is frequently no destination at all.
That is genuinely difficult for people trained to achieve postures. There is nothing to get right, which removes the thing many of us are using to stay interested.
What it is good for
Standing up. Not glamorous, but the honest answer. Sustained, unhurried, weight-bearing movement is unusually good at the unspectacular business of staying steady on your feet, and it makes very few demands on joints while doing it.
It is also one of the few classes on my timetable where being tired is not a reason to skip. Most of it can be done at whatever effort you brought.
If you are trying it
Give it three classes before deciding. One is not enough to get past the strangeness of moving that slowly in a room full of people, and the strangeness is not the class.
Wear something you can move in and stand comfortably in. You will be upright for most of it.