I would imagine huge number of non-technical users share a computer and want their own chrome profiles so that they can access their own emails without signing out of their family members. I know my middle-aged parents use Chrome in this way for example, and it would be a blocker for switching them to Firefox.

Is this basically a use case where they don't want to create separate Windows users for some reason, but still would want their own private space in the browser?

Honestly the elderly grandmother test is meant to model discovery for all users. Not actually use meant for grandmothers or even non-technical user.

Not grandmother but my non-technical dad cares about this.