Monitors play an intregal role controlling the overall volume when there's more than one performer, particularly if you're the person controlling the levels. If the voilinst is mic'd, but you are both going through the same PA system, a small monitor could improve your ablility to maintain good balance between the two instruments and vocalists. It will also add some volume to the room as well, especially in smaller venues.
Cheers,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)