Yeah it's for blood and oxygen supply and they suggest te left side because that's the side your heart is on and that makes it easier for blood supply. So your left side is best, right side is fine and your back is generally not where you want to be.
Hmm well crap... Sorry ftm here, lol good thing I found out sooner rather than later! I'm 23w now.
It's not a comfort thing, it has to do with blood supply going to your baby. If you lie on your back the pressure and weight of your baby cuts off good circulation. My doctor told me at 20 weeks you should avoid lying on your back for long periods of time.
As far as I've heard, it more of a comfort thing. If it isn't uncomfortable for you, go ahead. The reason they suggest the left side is if you are uncomfortable and having trouble sleeping.
I'm on my 4th pregancy and I can't sleep on my back at all so I always sleep on my side or on my tummy I know they say don't but I can't sleep anyother way but I normally do try to prop my leg up some