Hi Raveesh,
As Suhas Saha pointed out above, REFRESH is obsolete and I confirmed it doing an F1 on REFRESH and it's covered in Obsolete Internal Table processing section. It was new information for me.
The driving factor behind the difference among them is: efficiency of memory allocation. A developer assigns INITIAL SIZE to an internal table anticipating it to be the most likely the maximum size of the table. The system allocates it and keeps it reserved as long as s/he is manipulating table or even deleting all its rows and the system doesn't need to bother about it. But if the size of your table increases beyond that initial size, it's an overhead. And if you don't need the table any more, you can simply FREE the internal table to save the valuable memory space which the system can allocate to some other resource.
Hope this answers your question.
B/R,
Mukesh