If we are able to collect it, it can be used to flush our toilets. A stopgap would be to have buckets under downspouts and carry the bucket to the toilet when it is full. A large bucket can flush regular toilets two or three times. Water saving toilets could be flushed down even more times with the rainwater in the bucket. Why use potable bathable water for toilets, when we can use rainwater that falls during a season of rain.
To make it even more convenient, a pipe or hose with that taps into a downspout or a roof gutter can lead straight to a toilet. Additional considerations might be needed depending on the location and its climate.