By selecting any desired note and turning up its speaker (it can be LOUD !) you can actually ‘feel’ the resonance and ‘beating’ when going off the correct intonation. You can slide up and down from the correct intonation and really ‘feel’ it ‘come in’ when you are spot on. I wouldn’t recommend it just for this use because of its price but if you have access to one it’s a fine practice tool.
The advantage that the Boss has is that you can turn up the volume to the point that the fiddle body picks up the tone and then when you note the string the resonance is so strong that you can feel it in your fingers and ears, I can’t exactly explain the feeling but it certainly helps drone tuning practice. Too bad it’s so expensive now days.