Advantages of dynamic memory allocation in data structure?

The main advantage of dynamic memory allocation is flexibility: the sizes of structures (or upper bounds on the sizes) do not need to be known in advance, so any size input that does not exceed available memory is easily handled. There are costs, however. Repeated calls to allocate and de-allocate memory place considerable strain on the operating system and can result in "thrashing" and decreased performance. In addition, one has to be very careful to "clean up" and de-allocate any memory that is allocated dynamically, to avoid memory leaks.

The general rule of thumb is, if you can allocate memory statically, do it, because the result will probably be faster code that is easier to debug. But if you need to handle wide-ranging input sizes, then dynamic memory allocation is the way to do it.