"Known Enemy Unit" doesn't necessarily mean that your troops see the other guys. You and they are disposed in such a way that your fire has a greater or lesser chance of affecting your target. Concealment helps simulate this.
In other words, it's not a matter of seeing the infantry laying on the ground/hiding behind rocks vs. seeing their helmets sticking out of the top of fox holes.
Scenario VCs/VPs can depend on Fortification location so keeping them concealed can help keep the Attacker guessing.
Fortified building Locations remain HIP until their TEM is used or an enemy tries to enter the Location and you've got guys there, too. Otherwise, check the A12.121 Table because Fortification-'?'is lost pretty easily if you've got LOS.