The other answers are correct, but slightly miss the point.
The viewfinder help you to prepare the shot. It is (it was) not made to have exactly the same results as in the film/sensor. Now it is seldom, but on high end cameras you can change the viewfinder type: now as default we have the most neutral one, but in the past (and especially on film cameras) the viewfinders made much more evident the focus of image, making the rest of image more out-of-focus. The priority of viewfinder was and it is still focus.
So the priority of viewfinder were: focus, then composition. DOF had lower priority, and not really compatible with focus.
The Actual setting: full open lens helps to view and select the zone which are in focus (it has minimal DOF). Also as other answers: it is also more luminous, so it helps also composition.
With the DOF button press, you have less light, and an estimate of DOF: it is in any case an estimate because viewfinder is small, so your eyes and brain will see the image much more on focus as it is really [make a photos to a far/small text. You can read it when you zoom in, but at a certain point, when you zoom in it is more difficult to read]
It was also an estimate, because with old viewfinders, the out-of-focus parts were accentuated.
So with DOF-button you have some indication of DOF but not a definitive answer.
I also never seen any professional photographers to use DOF-button. It is good for student, to help getting DOF and so aperture by heart, as finally it should be.