When should I use malloc in C and when don't I?

I understand how malloc() works. My question is, I'll see things like this:

#define A_MEGABYTE (1024 * 1024)

char *some_memory;
size_t size_to_allocate = A_MEGABYTE;
some_memory = (char *)malloc(size_to_allocate);
sprintf(some_memory, "Hello World");
printf("%s\n", some_memory);
free(some_memory);

I omitted error checking for the sake of brevity. My question is, can't you just do the above by initializing a pointer to some static storage in memory? perhaps:

char *some_memory = "Hello World";

At what point do you actually need to allocate the memory yourself instead of declaring/initializing the values you need to retain?

char *some_memory = "Hello World";

is creating a pointer to a string constant. That means the string "Hello World" will be somewhere in the read-only part of the memory and you just have a pointer to it. You can use the string as read-only. You cannot make changes to it. Example:

some_memory[0] = 'h';

Is asking for trouble.

On the other hand

some_memory = (char *)malloc(size_to_allocate);

is allocating a char array ( a variable) and some_memory points to that allocated memory. Now this array is both read and write. You can now do:

some_memory[0] = 'h';

and the array contents change to "hello World"

For that exact example, malloc is of little use.

The primary reason malloc is needed is when you have data that must have a lifetime that is different from code scope. Your code calls malloc in one routine, stores the pointer somewhere and eventually calls free in a different routine.

A secondary reason is that C has no way of knowing whether there is enough space left on the stack for an allocation. If your code needs to be 100% robust, it is safer to use malloc because then your code can know the allocation failed and handle it.