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.
You don’t really need to worry about the volume size either. Too big a number just means you will have to work harder to get ranked in Google. It’s now quite commonplace to see SEO people say it’s best to just go with your gut feeling on a keyword. Just go for something you would type into Google search yourself instead of beating yourself up trying to find that ‘one’ killer keyword.
It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.