This tool was originally created to carry out keyword research for paid campaigns using Google's AdWords platform. However, it can also be used to research organic keywords, providing estimated global/local monthly search volumes for keywords (i.e. their popularity) and their competition (i.e. how difficult they are to rank for) for organic search.
Long tail keywords can find people who are later in the buying cycle, and more ready to buy. For example, somebody searching for “tents” is probably early in the buying cycle, just starting to research what they want. Whereas somebody who searches for “North Face Kaiju 4 person tent” already knows what they want, and is more likely to be ready to buy.
Understanding which websites already rank for your keyword gives you valuable insight into the competition, and also how hard it will be to rank for the given term. Are there search advertisements running along the top and right-hand side of the organic results? Typically, many search ads means a high-value keyword, and multiple search ads above the organic results often means a highly lucrative and directly conversion-prone keyword.
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.