Is search engine optimization necessary if my website already gets a lot of traffic?

That depends on what you want. If you want to increase sales, then yes. Traffic does not necessarily equal sales and can benefit from being better targetted for example.

I would say no, not strictly necessary. If you're already getting lots of traffic, make sure you have ways and means for that traffic to send you more traffic, such as a 'recommend this site' feature, or make it possible and easy for your visitors to bookmark your site with as many social bookmarking sites as possible such as StumbeUpon.com, Digg.com, Furl.net etc.

Also, make sure you update your site with relevant info on a regular basis and the search engines will eventually find you without any optimization at all. Congrats on having lots of traffic!

The short answer is: no. The goal of website optimization is to increase traffic by letting users find your site using keyword search on search engines. Hey if you have lots of traffic already, congrats!

Search engine optimization is the process of improving quality traffic via search engine. Search engine is still necessary because it is important to rank your site in the search engine and make visible to users. SEO may generate return of investment by promoting your site online. You can promote it through the link building strategies done by the seo such as one way link, reciprocal link, pay per click technique, paid inclusion and a lot more.

I would say traffic is not permanent. today if you gain adequate amount of traffic there is a possiblity that you might loose your traffic later. SEO is the technique that maintains the flow of traffic on your websites. Afterall, its upto you need to get into SEO or no, but I would personally prefer yes SEO is necessary.