The Tiberium silos can hold a large amount of Tiberium. If a commander captures a Silo, the amount of Tiberium that was given to the enemy would be lost and converted to the commander. Its fairly light armour makes it fairly vulnerable to enemy attack.
There is a way to bypass the need for building Tiberium Silos. Note that at the beginning of a mission, you are given an amount of credits which do not require any "storage space" in a Refinery or Silo. When you construct a building or unit and subsequently cancel it, the money is refunded to you in the form of storage-free credits rather than as Tiberium. Since the game prioritizes spending Tiberium before storage-free credits, this means that repeatedly building an expensive structure and canceling it will eventually convert all your Tiberium into storage-free credits and alleviate the need for Tiberium Silos. The same trick works in Red Alert 1 and Tiberium Wars, though not in Tiberian Sun (Tiberian Sun spends your storage-free credits first and refunds it to you in the same form).
Two different versions of the Tiberium silo appear in Tiberian Dawn; the version portrayed by the in-game sprite is distinctively dome-shaped, with some auxiliary structures adjacent to it, all set upon a concrete foundation, while the version used in renders and the cameo icon appears somewhat taller with flatter sides, no visible foundation, and a generally simpler exterior appearance. It's possible that the simpler-looking render/cameo version is an early model of silo initially used when the hazards of Tiberium were less well-understood, while the sprite version is an improved design that was used later in the war, specifically designed for improved safety of storage and efficiency of operation. This is possibly supported by the ruins of old TW1-era silos in Tiberian Sun exclusively being the sprite version, which may indicate that it had become the definitive design of the First Tiberium War, with the older render/cameo version having been phased out before the war's end.