You use dynamic memory when the size of your allocation is not known in advance only on runtime.
For example you ask a user to input names (lets say up to 10 names) and store them in a string array. Since you do not know how much names the user will provide (only on runtime that is) you will have to allocate the array only after you know how much to allocate so you will use dynamic allocation.
You can of course use an array of fixed sized 10 but for larger amounts this will be wasteful
