What is the usefulness of flange? Flange is used to positioning.

For example there is a fuel tank mounting flange installation is called, that is, for example a rectangular four weeks to play four symmetrical holes in the middle there is a big hole and cylinder diameter the same as this set the tank on the cuboid between them can be welded together by other means, tanks, and other things that depend on this connection is around four rectangular holes to connect, this can be called rectangular flange.