Symbols are generally thought of as relatively small repeating objects. So first of all you should use symbols whenever you need to repeat something, like sanitary fixtures, light fixtures, furniture, and so on.
But symbols also have some other reasons for being:
Even if you use the object once in a file, you might want to reuse that object on other projects. This would be a good reason to make the object and to symbol, because you can then use the Resource Browser to get that object from the library.
If the object is not a plug in object (like a 3-D object you’ve created yourself) you have to make it into a symbol in order to insert it into a wall.