Volcanologists can save lives (albeit indirectly) by determining how a volcano is likely to erupt - in the sense of what kind of hazards it might present (such as lava flows, lahars, pyroclastic flows, toxic fumes, etc.) and where those hazards are likely to strike. It is up to other agencies to use this information to develop evacuation plans and so on.
For example, one of the hazards if Mount Ranier goes is likely to be a lahar; evacuation routes have been planned to avoid the most likely route of the lahar, and one school actually has an evacuation bridge over a nearby ravine just for that purpose.
To a lesser extent, volcanologists can sometimes estimate *when* a volcano will explode; but accurate predictions usually have a range of only hours to days.