Use one as a backup when the main is in the shop.
Sometimes machines have different strengths so I set them up differently. My old Kenmore is great with heavy stuff so it gets used for that, my Juki does better with small seams or delicate items than my Janome. I have a new(to me) Bernina 230 that is the smallest and lightest so it will get used for classes and most likely to teach my DD to sew.