While free version of Keyword Tool provides only up to 10 autocomplete suggestions for a single search term, Keyword Tool Pro will give up to 19 keywords for one search term on Google and YouTube, 25 keywords on Bing and 50 keywords on the App Store. This means that you will get on average 2x more keywords for every search that you perform using Keyword Tool Pro.
These additional keywords, which are only available from Keyword Tool Pro, are keywords that you might not find using other keyword research tools and Google Keyword Planner.
Autocomplete predictions are automatically generated by an algorithm without any human involvement, based on a number of objective factors, including how often past users have searched for a term. - Google Help
Keyword Tool Pro will help you discover lucrative keywords with low competition that your rivals will not be able to get their hands on just by using Google's web interface, Google Keyword Planner or other free or paid keyword research tools.