For small towns tankers aren't luxuries, they're vital. Rural firefighting requires a much bigger focus on water supply, hence if a town only has one "pumper" they are only going to have 500-1,000 gallons of water to fight a fire until mutual aid shows up (in some areas upwards of half a hour) a tanker is needed to insure the firefighters a continuous water supply. This isn't like LA or NY where they have hydrants every block, the town could only be a few hundred people in a small area but the department would cover large amounts of sparsely settled land outside the town as well, and hydrants don't grow naturally on the prairies of Montana. To solve this problem many departments utilize pumper/tankers which carry more water, less tools and have large pumps (from the outside they look like any other pumper), most rural departments use only pumper/tankers as pumpers because the smaller tanks favored by urban departments just aren't practical. In short tankers aren't a required because its a volunteer department, they're required because its a rural department.