Why harvest the rain?
Why not would be my question, especially if your property struggles through the drier months when water consumption typically rises.
Yes its an investment but what is the true return? The answer to that will be different for everyone and dependant upon each persons situation and planned end use for the rain.
Rain harvesting can help alleviate the stress on low yeilding wells.
Reduce costs on municipal/metered water supplies.
Rain water can be used untreated for irrigation and outdoor use.
Can be treated to potable standard for stand alone systems or combined systems with proper air gap or backflow protection.