Also, removal of the formaldehyde and other chemicals used in textile processing thus making your fabric 'user friendly' and realigning the threads so the fabric returns to straight of grain thus making your cuts more accurate.

Why do you SOAK in Retayne BEFORE you put in a hot wash? I thought the directions say to WASH the fabrics in a hot water load for 20 minutes. I've never used color catchers....only use RETAYNE in a hot water wash. Never had problems then and no puckering of fabrics in my quilts because all the fabric is back to its original shape (before stretched when processed and bolted).

Another reason is to let the fabric relax into its "natural state," in case it was stretched and wound onto the bolt, crooked.

You can't always wash, especially if you purchase fabric directly from a shop where you have a class. But I always try to wash but a must every time is a colour catcher even when it has been quilted. Some people have been known to make a quilt from the colour catchers .

I don't prewash anything. I've been sewing for years, and never had any issues. If I ever do have an issue, I might change my mind, but I have so much fabric...I'd have to wash, dry, and iron for a few years to get it all done. Lol....that's not happening!! I've even done bright batiks with white backgrounds...no issues.
If you are concerned, you can take a little piece of the fabric and put it in a bowl of water to see if the color comes out at all. You can use color catchers too when you wash it. I have them, used them a few times, but found that our clothes bleed more color than any of my quilts have.

I do no prewash my fabrics and they have done quite well in the wash.

All you have to do to test for bleeding is rub a one inch corner of the fabric with a wet cotton swab or paper towel. If the color transfers, deal with it in whatever way you choose. If not, you're all set. At least you'll know, one way or the other, for the price of a Q-tip and a few minutes time.