The free subroutine frees a block of memory previously allocated by the malloc subroutine. Undefined results occur if the Pointer parameter is not a valid pointer. If the Pointer parameter is a null value, no action will occur. The realloc subroutine changes the size of the block of memory pointed to by the Pointer parameter to the number of bytes specified by the Size parameter and returns a new pointer to the block. The pointer specified by the Pointer parameter must have been created with the malloc, calloc, or realloc subroutines and not been deallocated with the free or realloc subroutines. Undefined results occur if the Pointer parameter is not a valid pointer.

most of the times you need to zero out the mem by using bzero or memset.U need not do it that way here.And also if it is to be alocated for a string,strlen(allocated array) is automatically 0.Which is a much safer method

while programming if the memory is allocated by malloc of calloc but free() is not called at the end
of usage of the memory chunk, it remains allocated..and can be dangerous..
these are very important in case of development of large products..
often programers do not intentionally leave the memory allocated..but there may be situation
when the control is passed to a different portion of code and needs to be freed there

Memory leaks are extremely dangerous...

The heap is where malloc(), calloc(), and realloc() get memory.
Getting memory from the heap is much slower than getting it from the stack. On the other hand, the heap is much more flexible than the stack. Memory can be allocated at any time and deallocated in any order. Such memory isn't deallocated automatically; you have to call free().