Volcanic-tectonic earthquakes (VTs) are, simply put, caused by slip on a fault near a volcano. Volcanoes are often found in areas of crustal weakness and the mass of the volcano its self adds to the regional strain. Most VT earthquakes have nothing to do with the magmatic system of the volcano but occur in response to regional strain exerted in an area of weak faults.
The volcanic tremor can be used to warn of an impending eruption allowing people living near the volcano can to be evacuated to safer areas. This method of prediction has been used successfully to predict the eruptions of Mount St. Helens in the 1980's and the 1991 eruption of Mt. Pinatubo in the Philippines.