how is prepatory course in iit

If you get a rank under 10 in preparatory courses you can get a good department. Preparatory course gives you the opportunity to explore the campus one more year and you are almost sure that you got into IIT .  This option is more secure than attempting once again for JEE. Even if you don't get a good department you can still try for department change after your first year.  Overall you get the tag of IIT.