Unlike similar tools, Serpstat is a page-oriented platform for in-depth competitive analysis. You can find competitors and define missing keywords for a single URL or even entire domains. You can also view historical position data for a range of pages organized by phrase, as well as see which pages have dropped in rank and their rank distribution as a percentage, which is very handy if you want to compare data from two different time periods or observe changes over time based on algorithm updates and other factors.
I think people's aresenal of keyword research tools are mostly the same: 1) You need a tool to examine search volume, most likely Google Keyword Planner 2) A tool to help you generate more keyword ideas. Tools that work with the search engines' autosuggestions are very popular such as KeywordTool.io and Ubersuggest 3) Then people might add a tool broaden the depth of their data, maybe including something like Google Trends or Moz's Keyword Difficulty tool.
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).