There are many reasons for doxxing - if the person being doxxed has done something horrible (pedophile, rapist, abuser, etc.), then putting their information out there lets others know who they are and/or to avoid them. It serves as a punishment. And to just say, "Look at this faggot, he touched a kid. Ew. Here's his address, get his ass."
Some people dox for a good laugh, although one could argue that putting seemingly innocent (albeit annoying) people's information online can be truly harmful. But we are all on this site to laugh at people, its kind of the norm.