Tiberium is used to build things, but it is also valuable as an energy source
According to the command and conquer wiki, Tiberium is valuable because it holds concentrated minerals and metals. These minerals were leeched out of the soil by the crystals themselves and are much easier to get to a usable state as compared to old-school mining and smelting of ores.
Emphasis added by me.
Tiberium leeches metals and other heavy minerals out of the soil, concentrating them in crystals which can be easily collected by specialized vehicles (Harvesters) and processed (in a specially designed Tiberium refinery) into easy to use resources. As such, it is an amazing opportunity for any economy. Harvesters (large, heavily armoured vehicles with specialised equipment) collect the crystal by sweeping it up from the ground.
For the first few games (Original C&C, Tiberian Sun, and Tiberium Wars), the useful materials refined from Tiberium were used to build structures and vehicles, and the industrial waste was sometimes used in weapons.
In the First Tiberium War, the Tiberium refining process began with the dumping of the harvested raw Tiberium into a Tiberium Refinery. The harvest would be transferred to a special centrifuge, where the minerals would be extracted alongside other useful materials, creating a thick, clay-like sludge that was transferred afterwards to the melting furnaces, where it would be molded into elegant bricks, preparing a clean, safe product for further use in Construction Yards, War Factories and other manufacturing centers. Waste and other byproducts would then be stored in barrels and stacked for removal from the refinery or, in Nod's case, some of it would see use in the chemical sprayers for the Chem warriors.
The situation changed after the Second Tiberium War (Tiberian Sun), when Tiberium was further transformed in the wake of Tacitus decontamination procedures. Now, the refineries melted the crystals as their first step in processing it into a safe, usable resource. This harvesting process continued into the Third Tiberium War (2047–2049) with few major changes, save for advancements in robotic extraction and new models of harvester, a notable example being the enormous MARV superheavy tank.
In later games (Tiberian Twilight), Tiberium was used for energy rather than raw materials.
The Tiberium Control Network (TCN) was basically one global network of harvesting operations that dissipated the crystals and then converted them into an inexpensive energy source. In this way, the TCN not only performed the duties of harvesting and refining all on its own, but it also contained Tiberium matter and prevented it from spreading outside of very centralized hubs. This advancement rendered previous methods of harvesting and processing obsolete and abandoned, and, in a way, marked humanity's mastery over Tiberium, allowing full control of the substance with minimal consequence.
Tiberium is valuable, but is it used a currency?
Probably not. Raw Tiberium is toxic and corrupts/kills living tissues, so it's not something that you would want to receive in exchange for goods and services. Processed Tiberium is safe enough to use to build things, but probably not inherently more valuable than the steel or alumnium or tungsten or other materials that would be used if Tiberium were not available. They still have value, but the value is not concentrated enough to be everyday currency.
As Nate Kerkhof pointed out in a comment, raw Tiberium is likely valuable enough to back a currency, much like the US dollar used to be backed by silver and gold. The currency traded day to day would not have the toxicity challenges of the raw Tiberium backing it.
Tiberium is useful because it is relatively time and energy efficient to go from crystals in the ground to building materials; once you have the building materials they are just that: building materials.