diatom material properties

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Silica has a number of properties that might be beneficial for an organism encased in it.

Hardness: a hard shell is good for protection. This might restrict growth, but diatoms also evolved a special reproductive method, the auxospore, to circumvent this problem.
Malleability: silica can be made into elaborate cell shapes or dangerous proterbances to ward off grazers. A rigid, malleable material is necessary to build many morphological features of diatoms, including areolae, spines, and setae.
Biosynthesis: probably not a significant advantage in modern diatoms, but silica templates have been shown to facilitate synthesis of biotic polymers -- DNA, polypeptides -- in the absence of biotic precursors. In other words, silica aids in the spontaneous generation of life, or at least large organic molecules.