I'm not sure I understand the benefit of covered terrain for unhidden units. Is an unhidden unit in a "cover" hex viewable in normal LOS of opposing unit, or must the opposing unit be adjacent to or in the cover hex itself? I can't find where in the the rules the meaning/benefit of "cover" is to anything but a "hidden" unit. Thanks.
"Cover" is used as shorthand for terrain giving a positive DM to a unit if attacked.
Besides hidden units which you mention, cover is also relevant for:
* if you use the optional rule 5.0.3 (foot unit cautious movement)
* 7.6.1 Rally dice modifiers
PS: if you have not downloaded the recently released new rule PDF file AtB-2nd-Edition-Rules-v25-ENG-sm.pdf I recommend doing so! Searching for "cover" in the PDF is a good quick way you can find this info.