cookies are temporary internet files. sometime you you 
access thepages which aresupposed to be viewed by you,say 
like your emails. if any wants to work offline that 
security must be maintained

hey please be specific yaar.i could not get you .please let 
me know why we must test cookies.

thanks.

Cookies are temporary internet files, which is stored in 
client machine...For instance in gmail you are entering 
username and password, so here username treated as a 
cookie, then it will send the cookie to all the pages which 
you are going to visit, finally click logout button then 
page will redirect to home or defalut page....Now the 
problem is cookies should destroyed from the machine after 
clicked the logout navigation or button, suppose the cookie 
does not destroyed, then what will happen, user clicks the 
BACK button from the browser without entering username and 
password in the login window, page will redirect you to the 
main page without authenticate....This is one of the use of 
testing cookies.....

If any mistake in this answer pls correct it....

Nicely said, Ooty! Thanks:)