I have noticed a few people talking about using washable yarn. What would be the benefits of using unwashable yarn? and does that make it dry clean only?
I assume they mean machine-washable, so it's not that the alternative can't be washed at all, it just has to be washed by hand.
I'm pretty sure that's what they mean, too. The advantage of machine-washable yarn is, of course, that it can be washed (and sometimes dried) in machines, so it's easier to care for. However, in order for yarn to be machine-washable, it often needs to incorporate acrylic or other synthetic fibers, which aren't always as nice to work with and may not look as nice or feel as nice in the finished product.
So the advantages to being willing to use hand-wash-only yarn is that you have more choice of fibers, and can use natural ones that don't do well in machine washing. Wool can definitely always be washed, but if you use hot water or agitate it too much, it can start to felt and it will shrink a whole lot. Silk can also be washed, but it shrinks if it comes into contact with heat while wet, and it's delicate. Other natural fibers can be machine washed, but it just depends upon what they are and what they are mixed with. In general, though, it's not the water that does damage to most hand-wash-only yarns...it's either the heat or the agitation of washing/drying. One thing I've noticed, though, is that a lot of the really high end yarns (that feel awesome, look great, are easy to work with, and/or have a wide variety of interesting color choices) just aren't made to be machine-washable.
Whenever I make things like blankets for people, I always put a priority on ease of washing, which means I often use acrylic yarn so it can be machine washed and dried. But for things like hats (which are easy to hand-wash and don't get that dirty most of the time) or clothing for people who don't mind hand-washing, I wouldn't pay much attention to how the yarn needed to be cared for when picking it out. In that case, the color or feel or ease of use for the yarn in question would matter a lot more.