Rainwater tanks are fairly easy to maintain overall, as it allows you to utilize a great, renewable energy resource that is extremely useful to all. Drinking water is not quite so easily renewable, so this really helps lower the wastage so prevalent in most societies.
Rainwater harvesting helps reduce the overall water bills, giving you a chance to be free of paying greater sums to water utility companies. This is extremely useful for a lot of smaller businesses and families, as it allows them to lower their overall bills. On a greater, industrial scale this works even better, as it allows harvesting rainwater in much larger quantities.
It makes it great for irrigation purposes as rooftops can work as an area for catching the water, which can then be linked to a harvesting system and water tank or rain barrel. This makes gravity the driving power here, lowering the use of fossil fuels.
It helps reduce the overall demand of ground water, as most cities take a serious toll on the drinking water of the areas around them. This greatly helps with the natural balance between our population density and the local biosphere.
It helps reduce erosion and floods, as rainwater can be contained in a way that doesn’t hurt the environment.
Rainfall can be pretty unpredictable in some areas of the world, so this could seriously inhibit the usefulness of a system like that. You should not depend solely upon rainwater for your water needs, especially in areas where such rainfall is limited.
The initial costs can be steep for some households, depending on the system and how much benefit can be derived from its use. Much like solar panels, the costs can be negated for the next ten to fifteen years, largely depending on the system itself and the rainfall of your area.
Regular maintenance is required for rainwater as the tanks can get mosquitoes, algae and worse if you leave them unattended for far too long.
Some roof types may be a problem for water tanks, as they can allow animal droppings, insects and even chemicals to seep into the water if used for plant irrigation.
You can also have a limit to water tank storage capacity (sometimes due to space available) that restricts the amount of water you can use. After all, during a heavy, pouring rain the system may not be able to handle the volume released by local weather.