What information is being conveyed about an answer by doing this?
Answers with very low net scores (negative -3 or below) as dimmed out as an indication that they have been judged not useful and thus not worth the attention of future readers. Anybody really interested is welcome to hover over them (which darkens them up) and read them, but the visual suggestion is that it's not worth it. In order to get a score like that at least three people before have had to read the answer and be willing to shell out 1 rep point (the cost of downvoting) in order to signal the community that a post is either not accurate or not useful. Downvoting can mean a lot of things to different people but no one person can dim a question, only a consensus will make that happen.
Net vote totals also affect the default sort order. The most useful answers (as upvoted by the community) will float to the top of the page. This is an indication that those answers should probably be read first and are most likely to be informative, helpful, and accurate answers to the question. The reverse of that is that the farther down in the list you get the less collective value has been seen in the answers. At the bottom of the list if there are negative scored answers that should be an indication that the community has seen a problem with them, and the -3 dimming effect is a kind of cut of to show that not just one or two people saw the issue but that there was actual consensus about the problematic post.
Remember not just anybody can downvote. It takes a small modicum of participation (earn 15 rep) just to upvote on this site. It takes much more (125) to downvote. This means that people from other SE sites that come over to this one with a 100 account association bonus can come out of the gate upvoting anything, but in order to downnvote you have to spend some time and contribute some posts and —hopefully— get a feel for how things work here. As downvoting also costs the voter rep, it's rare than new users do very much of it. Large numbers of downvotes on the same post almost always indicate the more established (and presumably knowledgeable) members of the site have agreed on the issue.