Complex sentence: Primarily, this is because the periodicity of atoms in a crystal--its defining characteristic--facilitates mathematical modeling, and also because crystalline materials often have electrical, magnetic, optical, or mechanical properties that can be exploited for engineering purposes.
Split into simpler sentences: Primarily, this is because the periodicity of atoms in a crystal--its defining characteristic--facilitates mathematical modeling. Likewise, crystalline materials often have electrical, magnetic, optical, or mechanical properties that can be exploited for engineering purposes.