A lot of people undertake Search Engine Optimisation with the sole aim of directing traffic to their sites, and completely forget that this is only half the battle. You can get millions of people on your site every day, but if none of them do what you want on the site, or worse feel your website is poorly created, then what's the point?
I.e. a vast number of sites are created for the purpose of commerce. So if you want someone to purchase from your site, you need to think of what is seen when someone clicks on your site. Do you have any calls to action? Is the site well designed, clear and obvious to its purpose? Can people find all the information they need? If the answer to these questions is no, all the traffic in the world isn't going to make your site successful.
Before you start a Search Engine Optimisation campaign, and really before you create any website, the first thing you should do is write down specific aims and objectives for the site. Do you want sales, sign ups, donations, to spread information, etc. Then once you have these targets, the design and style of the site can be created, before you even begin to consider how you will drive traffic to the site.
A lot of websites and search engine optimisation campaigns are also created to drive real world sales, with estimates of up to 80% of online searches being undertaken to research products being brought offline. Companies who optimise their website for this purpose might argue this is a moot point for them, seen as they are simply increasing brand and product awareness, instead of looking for online sales. I would argue the opposite. If someone is researching online a product to but offline, and don't like the look of your website, they are a lot less likely to purchase from your company in a store.