In a game with a lot of units running around, it's easy to lose track of the fixed and passive defenses. Stationary artillery, missiles, radar, and anti-air have obvious uses. Other elements, like concrete blocks and mines, can seem like afterthoughts, but they're handy for channeling and delaying the approach of an enemy force. As the online tips state, it's not necessary to place a mine in every square of the map grid in order to create anxiety in your opponent. Use a few scattered mines to divert attacking forces into an area covered by every defensive weapon you've got.
Concrete blocks are less subtle than mines, but they're harder to eliminate.