Re your asterisk, the justified true belief (JTB) criteria are considered necessary, but no longer considered sufficient as a definition of knowledge.
Because of that, JTB is often treated as a useful first approximation.
It's easy to see why the JTB criteria are necessary:
Belief: if you don't believe it then you can't count it as knowledge.
Truth: a belief is not (valid) knowledge if it's false.
Justification: accidentally getting the right answer isn't normally knowledge.
But the original claim for JTB was that it was a sufficient definition of knowledge. Later critiques like Gettier's showed that this is not generally true, i.e. there are edge cases for which it fails. In many scenarios, those edge cases don't matter much. So you end up with JTB being an imperfect but useful definition.
Comments
Re your asterisk, the justified true belief (JTB) criteria are considered necessary, but no longer considered sufficient as a definition of knowledge.
Because of that, JTB is often treated as a useful first approximation.
It's easy to see why the JTB criteria are necessary:
Belief: if you don't believe it then you can't count it as knowledge.
Truth: a belief is not (valid) knowledge if it's false.
Justification: accidentally getting the right answer isn't normally knowledge.
But the original claim for JTB was that it was a sufficient definition of knowledge. Later critiques like Gettier's showed that this is not generally true, i.e. there are edge cases for which it fails. In many scenarios, those edge cases don't matter much. So you end up with JTB being an imperfect but useful definition.