Outdoor griddles technically function as large skillets heated by a series of powerful burners. While you may use a griddle as a substitute for a frying pan on a kitchen stove to prepare all sorts of sweet and savory things, this thing is not a complete substitute for a burner. This means that putting pots and saucepans to boil water or make sauces is not a good idea.
Of course, in theory, a griddle will be able to provide the heat required for boiling a pot of water, but it is so slow and ineffective you will end up waiting for your water to boil well into the next day. The same goes for making sauces that you want to add to your grilled meat. All in all, it is generally much easier and faster to boil water on a regular electric hotplate or a burner stove.