Someone suggested me to put comments or reason along with the link, to which i want to disavow. Is this necessary to do this, is there any effect to it, or it is just a formality?
Yes, it is important to put the reason along with the link. Through this Google will know that why you want to ignore a particular link.
Well I used once or twice this disavow tool and I didn't added comments as per instructions present in google webmsater tools help.
Hmm, if the link is really that bad, you better disavow or remove it than get your site penalized..