Got these old ice cream tubs from a local ice cream place. $1 for a dozen. I wanted to increase my rainwater storage (currently have 2 rainbarrels). Realized I could stack these guys up as much as I needed. A few drilled holes and a spout off a cracked kombucha kit and bam. Homemade rainbarrel.
This is part of a set of daisey-chained barrels so I had to keep it to 3 tubs for height reasons. All told it only adds about 8 more gallons of storage but every bit helps. You could stack as many as you wanted though, within reason.
I kept it simple but you could also add additional sealing between the bottom-lid connection to limit loss that way. I will add a few extra pictures in the comments.
Not Op, but another reason why some people might collect rainwater is that the tap water where they live is way to hard. The mineral content is very high and for example most houseplants cannot survive on it on the long-term. My dad also used rainwater for his aquarium fish.
Yes, there are ways to filter it and make it softer, but it's just way easier to just collect and store some rainwater.