In order to know which keywords to target, it's essential to not only understand the demand for a given term or phrase, but also the work required to achieve high rankings. If big brands take the top 10 results and you're just starting out on the web, the uphill battle for rankings can take years of effort. This is why it's essential to understand keyword difficulty.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.