I think a basic name change is fair enough. Sometimes people think they have given away too much personal information, or that they have been objectionable to others and want to make a fresh start. I can't see the harm in that. Also people lose passwords etc, and can't log in to post under their regular name. Or they take a break and deactivate their account, so have to re-register. Or as mentioned, someone wants to talk about a personal matter without it being connected to their usual user name.
People deliberately changing name so they are not connected with their old ID and can claim never to have said something is a grey area, IMO. I can understand a fresh start, but if someone carries on where they left off but denies their history it is a bit underhand.
Sock puppet accounts are deliberately set up to cause trouble, and are different again. They can, of course, be traced if they share an IP (ie they are the same person posting from different accounts), but if they are different people communicating by text or messaging whilst posting separately it is more difficult, but not impossible to detect. And yes - people do do that, crazy as it sounds. When several apparently new people all claim to have been monitoring a site for a while and simultaneously decide to tell people what they are doing wrong, or 'make a stand' about something that was previously a particular poster's pet topic, it's pretty obvious.