Having access to running water is one of the requirements for comfortable living, and in large parts of the world this means being connected to a municipal water supply or a private well. However, some, in the interest of being self-sufficient, have installed a rainwater catchment system. Depending on the purpose for the system, it can be very different. It could just be a barrel for collecting water to use for irrigating plants and crops. It could also be a somewhat more complicated system that is used for flushing your your toilet or washing your clothes. However, if the water is meant for drinking, the water needs to be filtered and purified, otherwise, there is a risk of it containing pollutants harmful to humans.
The placement of your system is also very important, since the surface that funnels your water could contaminate the water. For example, if your roof has flaking paint on it, it is possible this contains lead that will end up in your water. You should also trim back any branches that hang over your roof, since this could make it so bird droppings end up in the water.