As example, if you put up on a keyword, your website will arise high in the search results for that keyword. You just pay for successful searches where you get visitors to your site so using a PPC engine can very cost-effective.
Marketing Plan - exactly how many sole traders/microbusinesses would bother to do this? Even if they did, things change so quickly that it may be a waste. Short, action orientated and ever evolving (but occasionally think about this stuff to see what's changed)! ;)