Becky at my LQS (Sunshine Stitches) explained it to me this way:
Use Retayne to prevent fabric from bleeding. Use Synthrpol (or other fabric catchers when your fabric has already bled. If you think a fabric might bleed, prevent it by using Retayne. If you have a 'surprise' of bleeding, Synthrapol will help remove the runs.
She always knows how to break down a matter for a dumb newbie!