According to an answer from the Meta on Stack:
SE doesn't just cater to the folks who ask and answer. SE tries to
cater to those who have a similar problem and find a post via Google.
Not all non-SE users know what the number [votes] means. It seems obvious, but
it may not be. Especially for those not familiar with Reddit/etc. So,
fading it [the answer] out is a simple visual cue "don't read this".
Stack does this automatically so users will understand that this answer has been downvoted and is very likely not to be helpful to anyone. It normally happens on questions from new users, as they don't understand all the guidelines of the site yet. But, everyone can still see these answers, and can still comment and vote on them, if they have the sufficient amounts of reputation.