Generating traffic is relatively easy but attracting high-quality visits is a very different issue. The success or failure of an advertising campaign strongly depends on the relevance of the advertisement to the target audience. An advertisement which is not attractive or relevant will not lead to an increase in brand awareness nor will it generate traffic. If the content of the advertising campaign does not match up with the landing page and with the offered products, then most visitors will leave the page immediately, leading to a high-cost campaign with only very few conversions.
Similar for natural search engine optimization – your entry in the search result pages needs to be perfect to attract converting visitors. People use search engines multiple times per day to find information or navigate towards websites. So if the pages of your website rank well then you will have a lot of sustainable, high-quality traffic.
In general it takes more time for natural Search Engine Optimization to have an effect on the traffic, whereas Search Engine Advertising is ideal to get quick results.