Two views of why God will judge even little things like every careless word:
Jesus stresses how much more severely the blasphemy of the Pharisees will be judged (Matthew 12:24). Since Jesus does good deeds, people should speak rightly of him. When their words instead reject him, they also reject his mercy. Such words will one day condemn them.
Every word that is of no use is not edifying and therefore should not be spoken. Jesus here implies that we can not meet this standard and so will be under judgment. On the great day of judgment each person will have to account for every spoken word.
Interpretation 1:
The meaning of Matthew 12:36 is clear from the broader context: that if even every single careless word will be judged, how much more severely will the blasphemy of the Pharisees be judged (Matthew 12:24).1 Jesus is here focusing on human speech, and in the immediate context, the careless word here refers especially to those spoken against his works and against the Holy Spirit. Since Jesus performs good, redemptive deeds, people ought to speak rightly of him. When they refuse and instead utter words of rejection, they place themselves beyond forgiveness. It is for such words that they will one day give account.2
Interpretation 2:
Every word that is of no use has no value, is not edifying and therefore ought not to be spoken. Jesus here clearly implies that we will not be able to meet this standard and therefore all be under judgment. On the great day of judgment each person will have to give account for every spoken word.3
36 Je vous le dis: au jour du jugement, les hommes rendront compte de toute parole vaine qu'ils auront proférée.