I prewash;
To remove chemicals, dust, bugs I don't want living in my house
To relax the weave of the fabric after being twisted onto a bolt so the grain will be as straight as possible
To remove any excess dye (after washing 4 times, it still bled when removing the water soluble quilting marks)
To see the real color and real texture and real weight (too thin, trouble ahead)
To shrink
To see how much it will wrinkle
I want to see the fabric, not what sizing does to fabric