Almost everything sold as a keyword tool falls into one of three groups, and confusing them is why people end up paying for the same thing twice. Some hold a database of queries and numbers. Some take a list you already have and do something with it: score it, group it, filter it. And some watch a result page over time. They solve different problems, cost very different amounts, and only the first is expensive to build.
Tools that hold data
These are the ones with an index behind them: a body of queries with volumes, difficulty scores and the pages that rank. You are paying for the index and its refresh, which is why they cost what they cost. There is a free member of this group that most people underuse, which is Keyword Planner, and a second that is not a keyword tool at all but answers the same question better for pages you own: Search Console tells you the queries you already appear for.
Tools that judge a list
The second group takes the list and helps you decide: scoring rows by what the traffic would be worth, grouping queries that share an intent, filtering out the ones you have no chance at. None of this needs an index, because the inputs come from wherever you got the list. This is the group Longtailo is in, and being honest about that is the point: it holds no data and it does not pretend the numbers you paste in are ours.
Tools that watch a result page
The third group checks positions on a schedule from a chosen place and device. Their value is entirely in the checking and the history: you are buying somebody else's machines doing something you could do by hand for ten queries and could not for a thousand. Search Console does a version of this for free for your own pages, with an average rather than a check.
Which of the three you actually need
If you have Search Console set up and a small site, you may need nothing in group one at all. Almost everybody needs something in group two, and most people do it in a spreadsheet that gets rebuilt. Group three is worth paying for when the list is too long to check by hand or when somebody else expects a report. Working out which groups you are already paying for twice is usually the first saving available.
Questions people ask about keywords tools
What is the best free keyword tool?
For queries you already show for, Search Console, because it is your own data. For anything else, Keyword Planner, which gives ranges rather than exact figures and is free with an Ads account.
Do I need a paid keyword tool at all?
If your list comes from Search Console and Keyword Planner and your site is small, often not. What you usually need is somewhere to judge and keep the list, which is a much cheaper thing.
Why do two tools give different search volumes?
Because each models the number from its own clickstream and index. Use one source across a list: the ordering of your candidates is what you act on, not the absolute figures.
Which kind is Longtailo?
The second: it scores and keeps the list you bring. It holds no keyword database and checks no rankings, which is why it costs a fraction of the tools that do.