They could have health challenges or physical disabilities. I learned from my local bike dealer that the main reason Pedego lobbied for the "Class 2" designation (instead of not allowing throttles, which seemed to be the first inclination in those writing ebike legislation) was because there are folks with knee, hip, problems, breathing problems, etc. who can't sustain pedaling. The idea is to make cycling accessible.
And, if you read the Disability and Ebiking forum, you'll hear lots of stories about the helpfulness of throttles.