I always get extra plants in case an element that I thought would work out doesn't and so I have more to choose from.
Just last month, I was planting a new lava rock container and plant arranging just wasn't going right; it just wasn't looking good. Then when I glanced at a reject plant that I placed down to get out of the way, I saw that a miscellaneous container it was sitting in looked fabulous with it. So then I abandoned the original container and started to work on that 2nd "accidental" container planting instead.
Here is the 2nd container planted:
