is this compulsary to give the 'tag' ot every discussion?

No, it isn't, but you can make your own reply searchable and add to the references to the discussion. Have a look at the existing tags. You don't need to duplicate them. If you bring up something relevant that isn't already tagged, add that.

you dont necessarily have to tag a discussion but its better to do so

many mylot members (includes me) have experienced that tagging with relevant words increase our earnings (directly or indirectly). And good tag words also help in searching a topic . Hence it is better to add relevant tag words with each & every discussion , response & comment .

it is not compulsory to give tags to every discussions but if u give tags then ur discussions are identified by the mylot . if someone starts a similar discussions then at the end of the page ur discussions can also be seen there. u can see this at the end of the page i.e. at the bottom of the page.

no it is not compulsary , but it is better to give , the reason is written under it ,, good luck