Because a Web page is of indeterminate width and height, the position of the top left is the only thing that you can take for granted. Putting essential elements there makes good common sense, but it's not the only way. You can put essential elements elsewhere on a Web page if you go to the trouble of making sure that it works as well, if not better than the usual. Standards' are very like conventions, they also make life easier, but in a slightly different way. Where conventions are handed down and accepted with little question, standards are arrived at by mutual agreement.
Conventions are a one way street, standards run both ways.
Because of standards, the Web pages we create can be viewed on a vast variety of computers. If the standards were more exact and more rigidly adhered to, the overall experience would be even better. Unlike conventions, standards are not counter-creative. Another close cousin of convention is 'fashion'. Fashion is a 'closed convention'.