People tend to wear one hat or the other, either SEO or PPC. However, in my personal opinion and through my experience over the years, if you are not leveraging both you may be missing out on some real opportunities.
When starting out and when on a limited budget, Search Engine Optimization and other various bootstrap techniques are the natural options. That is fine and if you are in this stage, do not feel pressure by this post to get into PPC right away (although, listen closely).
One of the benefits of SEO is that not only gets us rankings under targeted keywords, it gives us a good deal of insight into keywords we rank under. If you rank under a keyword and you are seeing conversions, the next natural progress is...
PPC campaigns.
This may seem like having two fishing poles in the same "ice hole" and to a certain degree, that is what you are doing. It is a "doubling up" approach that can lead to a significant boost in conversions.
If you know a keyword converts, why not get more traffic to it?
It only make sense.
Let's take a quicke example. Say I rank under "the best purple widgets online" and I am able to get ranked #3 in Google under this term. Say I see some conversions coming in at a rate of 1:15 visits.
I can safely assume with a very small standard deviation that I am now going to be able to convert at 1:15 if I get paid campaign running in Google Adwords. I can basically double my presence and my overall revenue by getting an ad up under this keyword (assuming that it has a positive ROI at 15 clicks at the price per click).
I can then make my entrance into Bing Ads targeting this same keyword.
I can even run a test with Facebook Ads under this term.
Moral of the story, if you have a keyword and it is converting, do not stop with SEO. Move in the PPC market (under that keyword "exact match" to begin with) and you will instantly see more profits and more money in your pocket. That is the ultimate goal right?