There are other methods of storing water (dams/reservoirs, etc.), why is a water tank better?

With unreliable water sources continuing to be an ongoing issue, using water tanks can help reduce pressure on depleting water sources and prevent your stored water from being subjected to high evaporation rates and seepage.

The goal of farm water planning is to have the amount of water you need, where you need it, when you need it. Many farmers rely on a variety of methods for ensuring they have enough usable water even during the dry season. These can include dams, water wells, surface water storage and natural ponds.

While these methods may work fine if the weather behaves predictably, surface water storage is very inefficient and often has a high evaporation rate. During extended periods of drought, surface water storage may dry up completely. Using an enclosed poly storage tank ensures that there is no loss of already-collected water, and that it is kept clean, safe and ready for use. Additionally, carted water is best stored in water tanks where it would not be subject to high evaporation and seepage.

Also see:

The Problems with Dams on Farms That You May Not Know About