What is the use of opt and ini file?

Ini file is specific to that project. Instead of creating the two project like IE6.vtp and IE7.vtp, user can reset the default options through the opt file.

Another way of looking at option files is to look at the partner.ini file. This file contains all of the options (Agent, Runtime, Extensions, ect.) that get loaded when you start up SilkTest. An .OPT file is an .INI file that contains items from the partner.ini file that you want to have changed when you open that option set.
 
Why? -- Suppose you have 2 projects that you work on. Each have different extensions being used and frame files used. Instead of manually typing the frame file into the Use Files part of the Run time Option dialog each time you want to switch to testing another app, you could have all the options you need stored in an .OPT file.

  To open the .opt file
    1. open the silk test
    2. Navigate to the options menu and click the open new options set.
   
  For Example:
    Excel 2003 and 2007  -- Change or reset the extensions through the opt file.