To me, they exist to let you check the framing and composition. That was always given as the big advantage of the SLR design over the rangefinder - what you saw was what you got.
And from that - the viewfinder should let you work on seeing what was going to be recorded. Not add distractions.
Camera makers thought otherwise. First the aperture, then the shutter speed. Then all manner of other things. And all the while in generally a small viewfinder. For goodness sake, why a small poky viewfinder? Ah, well you get small viewfinders because the APS-C sensor is smaller, don't you? That's why it's smaller than the old hat 35mm SLRs. Fiddlesticks! I've looked through APS-C viewfinders which are small; but the really small Olympus E3 has a viewfinder that gives the same size image as my OM1 - which was reckoned to give a large image in its day. The only reason for a small image viewfinder is cost and size.
Do we really need to have shutter speed, aperture, ISO, blinkies for over/under exposure, grid lines and spirit levels on show, all keeping our attention away from the actual scene? I'll make an exception for grid lines, as I have them on my usual camera (Walker Titan SF 5x4) and I can ignore them; but then the screen is bigger.
What do you use your viewfinder for?