I used to wonder why anyone needed more than one. Now...my first one was a Janome Memorycraft 4800 which is great for piecing and walking foot but useless (to use a politer word than I usually do) for FMQ. So I bought a 7700, which is fabulous for quilting. I kept the 4800 because of its other strengths, in case I wanted to work on both piecing/walking foot quilting and FMQ at the same time. People on here talked so much about Featherweights that I gave in to my curiosity and bought one. I fell in love, and that gets used for making blocks and piecing smaller projects. Later I found a Centennial FW in a local junk shop for a very low price - I still have it but may sell it at some point. And before I gave in and bought the 7700, I was looking for a cheaper solution to the FMQ issue and tried a Singer 301. It didn't solve that problem, but I've held onto it because it's brilliant for any heavy duty work - eg sewing together several layers. When my Mum died about 18 months ago, my sisters gave me first refusal on her very solid 1960s Husqvarna, and I was tempted, but had to admit that I didn't need it, so my sister has that - and there's a little bit of regret there...but she's more likely to use it.
And that's how I went from "why would you want more than one machine?" to 5!