PPC is not a direct replacement for SEO, and vice-versa. In fact, PPC advertising can compliment an SEO strategy by providing more opportunity for your listings to appear on the search engines.
If your website is already ranking page 1 for a desired keyword phrase, and you were also present in the Ads section of the first page, a user may be more inclined to click on one of your listings due to seeing your business name multiple times.
Not only can PPC ads help with brand recognition, but they can also provide you with top three listing coverage that you would not be able to get without paying for it. For those users out there who do click on the first adverts that they see when searching on Google or Bing, being visible at the top of page can help to convert these users into potential customers.
In the early days of an SEO campaign, it may be recommended to run a PPC campaign to help generate traffic to your website until the effects of the SEO kick in.