If I have an infinity focus lens why the need for glass in an adapter?

If I use a m42 to Nikon body adapter and my lens's focus to infinity why would I use glass in the adapter, another "filter" of light between the lens and mirror? I'm an old school photographer and have never had to deal with this "big deal" everyone is making of infinity focus it's just a normal thing thats a part of everyday photography. I'm just wondering why these adapters would need the glass in them, it seems pointless but maybe I'm wrong.

I guess you don't (need the glass)

It's all down to the distance from the sensor.
Now I use m42 lenses on a canon, and our adaptors don't need glass, because the distance between the sensor and the lens is slightly smaller than it used to be on the cameras designed to take these lenses, so the adapter just holds the lens at the correct relative distance from the sensor.
With a Nikon the distance is bigger, and as you can't shove the lens into the camera far enough to hold it the correct distance from the sensor (the mirror would be in the way etc.) you need glass in the adapter to focus onto the sensor.
It would be the same for me if I got an adapter for FD mount lenses.

The infinite focus issue is related to the quality of the adapter, you may need to add tiny shims or file it down to get the EXACT distance from your sensor, otherwise your infinite focus won't focus to infinity.

There's far more info in Flickr groups for m42 lens users.