From the above replies,you would have now understood multiple information on no-follow links and its usage. But I have few question to you on your post:
1) What did you mean by, 'my website still getting good rank' ? Keyword Ranking or Google PageRank?
2)On What basis did you think that your website's achievement is because of lot of no-follow links to your website?

If you can clarify these questions, I can give you very specific answers to you.

Nofollow is helpful in driving the traffic towards your website, so it is one kind of advertisement....

Not really, nofollow links are not worthless. Maybe search engines like google don't count nofollow links as links. But nofollow links can bring high traffic to the website. I guess this is the only advantage of a no follow link. Sometimes these no-follow links are quite useful.

Nofollow links are not counted for the page rank but they are good for the traffic, so we can't say it is worthless

I don't think no-follow links are worthless. No-follow links can't pass link juice but can drive huge traffic to the website.

Of course it is worth less as compared to do follow because do follow not only attract traffic but counted by giant Google. No follow only done former job.

Of course it has worth less than do follow as later is count by Google.
However Yahoo count both for page rank improvement.

No follow link are not good as Do follow links. No follow links are only good for the build the visitors. not a backlink.. because Google not consider the no follow links. If you have good result form no follow then try to do in a high PR links.

No its worth.... as it helps our link building method look natural.