The reason I sometimes use this facility with the GKP is to find volume amounts for the keywords I’ve chosen. But, these days, it’s not so important. As long as the Long Tail Keyword makes sense and is something I would put into a search engine, then it’s good enough. Sometimes the most blazingly obvious keywords are often over looked by eager, trying-to-be-clever marketers. Sometimes you need a tool to highlight them.
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).