In the early days of Wikipedia, renamings took place manually, using cut and paste, before the move page function was enabled for non-administrators in August 2002.
Cut-and-paste moves still occur today because of unfamiliarity with the move function, unawareness that attribution is necessary, or when the move function fails (e.g., because the target has history) and people don't know to use the Requested moves forum to start a move request.
When a cut-and-paste move is done, the page history of an article or talk page can be split among two or more different pages. This is highly undesirable, because we need to keep the history with the content for copyright reasons. (See Wikipedia:Copying within Wikipedia.)
In some circumstances, administrators can fix this by merging page histories, using the procedure given below.