Water tanks can be a good hiding spot for rats and vermin if kept close to the ground. To keep pests from nesting in empty slimline water tanks during the dry days, they are elevated at a height to avoid entry of vermin.
Elevated water tanks are also less prone to leakage as they are made sturdy and strong, and any leaks are easily visible and can be taken care off. Suppose a certain section of the water tanks has corroded and there is a slowly forming hole. Water escapes and drips down making a noticeable puddle. It alerts homeowners that their water tank is in need of repair.
Water tanks at a certain height make it difficult for someone to poison or contaminant the water. There are many crazy people in the world who like to cause harm, and an easily accessible water source can be used to settle a score or for an evil prank.
The water is also safe from hazardous spills that could happen because of a overturn truck carrying poisons or someone spilling a barrel of chemicals. You can protect your home, but such contaminants can seep into the ground and accumulate, eventually percolating into your unprotected water source.
If the water tank remains at an elevated height, then there is a very small chance of an accidental puncture due to a crash with a vehicle, object or a sudden situation wherein sharp projectiles hit the cover.