Simply put, the concrete wall has seen service as a defensive barricade for almost a century. Simple, reasonably priced, and vastly superior to sandbags and Barbed Wire, concrete was the main static defense for many bases during the First Tiberium War. They are vulnerable to explosive and cannon based attack, but their thickness barred C4 from effective use against them.
Derived from civilian standard design, the standard concrete walls of most of the wars were fairly tall, preventing enemy infantry from climbing over and stopping direct fire weapons, such as tank cannons, from hitting a target behind it. Steel rebar inserted during construction give it the strength to withstand multiple hits from a tank assault. During the First Tiberium War, these were virtually the ultimate in defensive barricades, being able to halt the progress of any unit, even tanks, for some time.
In the Second Tiberium War, laser fences and the Firestorm barrier proved to be more powerful than the old mainstay, but the fact that concrete did not need power to operate made it an attractive supplement to energy-intensive barriers. Walls could contain the spread of Tiberium somewhat, acting as a barrier to its growth on the surface. GDI and Nod have distinctive wall designs. Their construction method in this era is somewhat different as, rather than build one section at a time, they are constructed by placing two endpoints between which a segment of wall is erected. The cost associated with the construction is determined by the number of points designated, rather than the length of the wall, to a certain degree. At this time, GDI defensive structures were merged with the wall itself.
In the Third Tiberium War, walls had fallen out of general use; however, massive walls were still in place to protect vital positions, such as Sydney and Temple Prime. Otherwise, walls are not considered a viable option as they are considered too expensive, management intensive, and nearly useless in the presence of more advanced technologies.
During twentieth-century wars, concrete was unrivaled as a barrier material. Though slightly more expensive than its competitors, concrete could stop tank shells and halt the progress of any unit. During the Second Tiberium War, it was far more economical than other, high-tech fortifications.
Despite being able to stop direct-fire weapons, artillery and other indirect-fire weapons could pass over the walls and strike their targets unhindered. Even more so, aircraft were totally unaffected by the presence or absence of walls. Also, despite its superior defensive power, concrete was more expensive than sandbags or barbed wire. Finally, in later wars, laser fences and the Firestorm barrier had significantly better performance, though this was offset by expense and power demands.