My dad's cancer spread to the bones in the spine and rib cage. He told me it was incredibly challenging and exhausting to stay still during scans as it just hurt so bad. Several times they had to redo scans as he moved.

I can certainly imagine there are many similar scenarios where it's just not possible for a patient to be completely still, or it's better for them to avoid excessive rescans.

A lot of people get panic attacks / claustrophobia and are incapable of getting in or staying still for a scan.

I have a lot of sympathy. I'm pretty good in confined spaces usually, but even after multiple MRIs it's still a surprisingly stressful experience. The buildup and safety questions make the pre-experience worrying. The aperture is surprisingly small. Depending on the scan, part of you might be caged in place, and it's extremely noisy and you're aware of a lot of mass and power spinning very close to your face.

Also, some of the radiologists don't help. It's not deliberate, but they're entirely desensitised to the experience (and often haven't actually gone through it themselves; which again seems crazy considering the lack of radiation). My last scan was of my lower back, but they were already set up (from the previous scan) to feed me in head first rather than feet first. From their point of view it saves a bit of faffing with the software and moving the pillow to the other end. From a patients point of view it makes all the difference in the world; it's a very different psychological experience having your legs inside with your head free, vs being stuck head first in something and having it whizz past next to your head.

I've had a goodly (read unhealthy) number of CT and MRI scans and I'm bright enough to understand which one is significantly more detrimental to my long term health. I'm also aware that on a subconscious almost cellular level, it's the benign one that absolutely terrifies me every time...

To add to the sibling comment, being stuck in a small, incredibly loud tube usually pinned under some receiver isn't great for claustrophobic patients either.

Critically ill patients, animals, children/babies.