Datamining in online poker is to gather informations about played deals (hand histories) in which you did not participate. Ie of the players who took the place, who is a blind, hole cards in cases where a showdown or a player they showed who the actions performed during a hand, etc. - is the same information that are stored in a special folder your poker client during your game and you are importing into the program for analysis - Pokertracker, HoldemManager etc.
Why is it needed? - It's simple - you see the style of play and the opponents, can each of them to play in the best way. For example, if your opponent statistics rarely makes a preflop re-raise, after his re-raise pre-flop, you can safely fold your jacks, and if he does so often, your jacks are often stronger than his cards.
Who needs it? - Those who know how to use it. If you play just for the fun of it is likely that you do not want mining, with luck you can win without statistics.
And if you make or plan to make money playing poker, then of course you need datamining, need to find a table with fish, to understand exactly how to beat this particular fish, how to beat or not lose encountering strong opponent, etc.
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