I have more than one going too,for the same reason as some of the others stated - I get bored, or a problem has arisen on something - then I need to do something else while I think about the problem. And, some is hand work, so that is by my chair so that I can do it while I watch TV at night.
Wish I could stick with only 1 at a time - But, that isn't me - have to be doing more than 1. LOL