confused on Definitions

I am just trying to understand some definitions. When would you use Offset or Index instead of using specific cell references? From the Microsoft definitions:
Index:
INDEX(array,row_num,column_num) returns the value of a specified cell or array of cells within array.

INDEX(reference,row_num,column_num,area_num) returns a reference to specified cells within reference.

Offset:
Returns a reference to a range that is a specified number of rows and columns from a cell or range of cells. The reference that is returned can be a single cell or a range of cells. You can specify the number of rows and the number of columns to be returned.

Thanks in advance for your help!