If you don't, you end up with a partially felted blanket/project. (Felted portions have raspberry dots next to them to point them out.)
I was going to write a yarn review, because I SWORE this yarn claims to be superwash. Before going off though, I decided to check the Ravelry page, which led me back to the page from the online store I purchased it from. Sure enough, it says it is not superwash as I believed. So there goes my ammunition.
This is Yarn Love Anne Shirley and was washed in icy cold water on the gentle/handwash cycle. Yeah. Not superwash at all. In fact, I'm wondering how it would have fared being handwashed.
The colors are gorgeous. The yarn is lofty and soft. Even the felted sections are lovely and snuggly. Since the washing mistake was mine, I can't think of anything negative to say about the yarn, since I should have known better. I didn't even run into any schlubby bits in the skeins.
So there you go.