I have used rain water for cleansing, healing, & rejuvenation in spells. 
I generally only use storm water as an offering to my deity. Storm water 
has been associated with strength & power due to the extra energy contained within it from the storm. It would be good for strengthening spells, protection, & kind of super charge of your intention for anything you cleanse or consecrate with it.
I’ll see if I can find some more information on them for you. I believe we have a spell using rain water on the site. I’ll see what I can find 
→ Sending Pain Healing to Someone
Others may have used them for other things too & will let you know when they are available 