I use spray baste for some quilts and wash them to get it out.
It also washes out any starch used to stiffen the fabric.
I also wash to remove any markings. I use a blue water erase pen to mark complicated quilting lines. Even if i spray it with water, there always seems to be blue markings left behind, so I wash in cool water to remove them.
As some of my quilts are old UFO's, it is wise to wash out any accumulated dust and dirt.