Keyword Researcher is designed to search the Autocomplete databases of Google, YouTube, and even Amazon.com. That's a lot of keyword sources! As you probably know, if you're building an authority site, or if you would like to become a dominant player in your niche, then starting with a large database of keywords (that your customers are actually typing in) is a powerful marketing tool.
Using the good ol' free Google Keyword Planner is great. But there is an alternate universe of Long Tail keyword phrases that live in Google's other database--namely, their "Autocomplete database." That's the database that is accessed when you're typing queries into the Google search box (notice that list of suggested keywords that pop up as you type?)
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.