When I was growing up as a kid in my home town of Selkirk, Manitoba, LOTS of people would do it.
You simply position a "rain barrel" under the downspout from your roof's gutters to collect the rain water that falls on your roof. The rain barrel behind our house was about 4 feet in diameter and about 6 feet high. It sat on a bed of railway ties, and there was a spigot at the bottom of the rain barrel that you could connect a garden hose to. My mom would use that water to irrigate the garden she grew behind our house. When she pulled up the last of the vegetables she grew in the fall, my dad would keep collecting water in that barrel during the rains we got before the cold weather came in October and November or until it got to below freezing at night. He connected that spigot to our downstairs toilet through a basement window so that when we flushed that toilet, the tank would be refilled with rain water from the rain barrel. Both irrigating the garden and refilling the toilet were done entirely by gravity.
I think if you ask anyone who's over 40 and grew up on the prairies, they'll ALL remember the rain barrels that people used to have behind their houses for growing their gardens. I expect that if you go to any garden center where they sell seeds for growing vegetables, you could probably still buy a rain barrel.
We also had a "burning barrel" behind our house for burning our garbage in, but as the town grew, they passed a by-law outlawing them because the sparks from them would cause grass fires on a windy day. I myself had a bit of a scare when I was burning some stuff in our burning barrel and a small field behind our house caught on fire. I ended up stamping the small fires out with my shoes, but I had a hard time keeping up with them, and I got a good scare that I wouldn't be able to get the fires under control and that this situation was gonna get out of hand. It's those kind of emotional moments that you remember your whole life. I was only about 9 or 10 years old at the time.
We lived in an older area of town where most people had gardens beside or behind their houses, and it seemed like it was normal to have a rain barrel. It's only when I got older and made friends in school that lived on other streets that I met kids that didn't have rain barrels or burning barrels.
I remember that my mother had a bug screen over the top of our rain barrel and the rain water poured onto the screen and through it. I understood at the time it was to prevent both bugs and dandylion seeds from getting into the water.
You don't see rain barrels behind houses anymore, and I'm not sure why not. I can't see any problem collecting the rain that's going to fall on the ground anyway, and use it for irrigating the ground in a more advantageous manner.