Cover means information about your position is leaking to the enemy. Who can exploit that intel in any way they find favorable at any time. They can see if you are moving or planning to move, they can assess your numbers, armaments and capabilities, your readiness and any damage, vulnerabilities or wounded members, they can look for resupply or reinforcements that you will need that are inbound to your location and mess with them, lots of options.. for which an assault on your cover/fortified position is not even necessary and may be counterproductive.

Not so much for concealment. An armored assassin is going to play second fiddle to an invisible assassin almost every time. Battles are won by exploiting fog of war and surprise attacks from unexpected directions or positions all the time, and thats a concealment option.