Why structures need internal padding?

It's for ``alignment''. Many processors can't access 2- and 4-byte quantities (e.g. ints and long ints) if they're crammed in every-which-way.
Suppose you have this structure:
struct {
char a[3];
short int b;
long int c;
char d[3];
};
Now, you might think that it ought to be possible to pack this structure into memory like this:
+-------+-------+-------+-------+
| a | b |
+-------+-------+-------+-------+
| b | c |
+-------+-------+-------+-------+
| c | d |
+-------+-------+-------+-------+
But it's much, much easier on the processor if the compiler arranges it like this:
+-------+-------+-------+
| a |
+-------+-------+-------+
| b |
+-------+-------+-------+-------+
| c |
+-------+-------+-------+-------+
| d |
+-------+-------+-------+
In the ``packed'' version, notice how it's at least a little bit hard for you and me to see how the b and c fields wrap around? In a nutshell, it's hard for the processor, too. Therefore, most compilers will ``pad'' the structure (as if with extra, invisible fields) like this:
+-------+-------+-------+-------+
| a | pad1 |
+-------+-------+-------+-------+
| b | pad2 |
+-------+-------+-------+-------+
| c |
+-------+-------+-------+-------+
| d | pad3 |
+-------+-------+-------+-------+