Why Would Someone Choose a Frameless Pack?

https://outdoorsfaqs.com/why-would-someone-choose-a-frameless-pack/
Here are some possible reasons for choosing a frameless backpacks:

Body shape not suitable for a framed design.
When you want to have your hips free for a wide range of movements.
When you use a climbing harness.
When you do not want to have a hip belt.
When you carry a small day pack with not much weight.
Frameless packs may be more affordable.
Frameless packs may be lighter.

There may be more reasons, but in any case, those presented in the list are discussed here.