Well building anything with tubes makes no sense at all but that doesn’t rule out making headphone amps. Many small devices are unable to drive many kinds of he

Well building anything with tubes makes no sense at all but that doesn’t rule out making headphone amps. Many small devices are unable to drive many kinds of headphones so you need something to bridge that gap. The other reason you might have is devices that can drive your headphones but have a lot of coupled noise from nearby digital electronics. This goes for most laptops. As an example I have a Zoom F4 portable recorder. It has a headphone output but it can’t drive 30-Ohm headphones worth a damn and even with high-impedance headphones there is audible noise that doesn’t exist on the recording itself. So if I want a realistic feedback on what I am recording I have to monitor through an external headphone amp.