The Watch List is nothing but a compound of those shows, movies and TV Series from HBO library that you don't want to forget watching. You can add an unlimited number of shows to the Watch List and classify them by expiry date, title, or according to whichever order you like. You can choose from your Watch List whatever you feel like watching in a given moment and then take it out of your list or keep it as a permanent reference for you.
The titles within your Watch List will remain the same, even when you register in HB GO from a different computer or device; that way you can make sure that you have your Watch List available everywhere you go.
The shows will remain in your Watch List until you decide to delete items from your Watch List, or until their expiry date.