It basically puts the reply below the original poster reply so that people can more quickly see the answer to a question. Take for example, if we had a long winded discussion where I was investigating an issue but it wasn't until the 10th reply that you received the answer you had been waiting for, by accepting it as the answer you are making it go near the top for everyone else to see. In addition to this it's also a curtsey to the forum mods by showing in the main category that it's resolved, this creates one less extra check in terms of looking at threads that need to be addressed.