Because there's so much fake news out there. On both sides. Having someone independent say look this is the facts helps the users.
Fact checkers don't check the facts as posted. They look at it through a biased lens and call it false even though their rationale doesn't match what the post said.
Example (REAL): Trump said the FBI spied on his campaign.
Politifact: FALSE. After many investigations by inspector generals and others, there is no evidence that the counter-intelligence investigation involving the Trump campaign was politically motivated.
Wikipedia: Spygate (Conspiracy Theory) Trump stated that the Obama administration put a spy in the Trump Presidential Campaign for political purposes.
Notice they went back to "for political purposes", even though that's not what Trump said? They didn't argue a spy wasn't placed in the campaign at all, just not for "political purposes".
This is the game the so-called fact checkers play. They use, or add, a couple of words that give them justification to declare the entire statement or idea false, even though those words were not part of the original quotes.