ANSWER - Rain can have a lot of uses in magic. The most common is that it is used by many witches whenever water is called for in a spell. A lot of witches dislike the idea of the chemicals used in tap water being incorporated into their workings and prefer to use the water straight from source.
It is important to note that there are different types of rain and that each type may be used to bring differing energies into the magic being worked. Rain from a warm gentle summer shower could be used to encourage tranquillity and the sense of sleepy summer days into a spell whereas rain from a powerful thunderstorm may be used to invoke passion and strength. As a word of advice I would suggest if you intend to work with rain it is well worth documenting the nature of the weather and time of year on the day you bottled it so you can recall which characteristics you are hoping to bring to your magic.
Rain and water in general are commonly used in cleansing rituals and spells. This can be a physical cleansing (think immersing your non water soluble crystals in a bowl of rain water), it can be a spiritual cleansing (think anointing your forehead or washing your face with rain water to connect with the element) or it can be a symbolic cleansing (think bathing a poppet in pure rain water to watch away negativity).
I’ve undertaken effective spells where I have used rain water to wash away ink on parchment to perform a spell to reverse an action and similarly this method could be employed for memory spells – if you want somebody to forget something you can write it down in non-waterproof ink and then simply agitate the paper in a bowl of rain water until the writing has dissolved away.