Maintain eye contact in meetings while texting in the front pocket of your hoodie, of course.

But in seriousness, if you have a smartphone, you already have access to a terminal, a relatively powerful local processor, connectivity and plenty of remote options, but it’s also likely much more expensive than a pi. So the reasons are IMHO:

1. Hackability (full access to hardware capabilities)
2. Physical keyboard (assuming good build quality)
3. Relatively low price
4. SFF native linux
5. Novelty/cool factor

Edit: answer seriously