Academic CVs can get quite long, listing conferences presented at, committees served on, etc. It's easy to see that some people would feel that the whole world doesn't need such a detailed record of their life.
- Actively ask another researcher for their CV,
I think it's more likely a non-academic would need this sort of thing.
Here are some ideas why you might need someone's CV:
- A journalist or university PR person reporting on their work
- Preparing a biography (e.g. a conference introduction)
Or, as an academic, you might really like someone's work, and want a quick way to find all their papers. Google Scholar isn't perfect, especially when people don't curate it. Personally, I wouldn't email for a CV in that case, but someone else might.
Or perhaps they are open to being recruited by another lab or department or company, but want to know when someone is interested, rather than not knowing. Is this effective or a good idea? I have no idea.