The electric griddle provides an extra cooking surface that can be the size of two regular skillets or larger. It's great for cooking things like pancakes, french toast, lots of bacon, fajita fillings and just about anything else you'd cook in a skillet.
If you have the kind that has a heated lid (similar to George Formans) then you can do pressed grilled sandwiches (like panini).
The good thing about it is that it will maintain a set temperature automatically. There's no need to fiddle with burners.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan