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 the OP so can't say why they're doing it, but in the UK at least it's really common practice for people who grow gardens because we'll frequently have "hosepipe bans" which means we can't hook our hoses up to the mains to water our gardens. So, you either have stored rainwater (like OP is doing) or... a very dry crispy garden.
I don't know how much it saves money wise, but ecologically using water from rain instead of the mains offsets the amount of water that needs to come out of treatment plants. After all, plants don't care about water treatment really. So they don't need the fancy drinkable tap water. They can have rainwater and that means more for the humans.
Thanks for the insights, didn't know about UK hosepipes bans.