The protein shell of viruses provides maximum internal volume in the most economical conformation by adopting icosahedral symmetry.
"There are several reasons why viruses adopt icosahedral symmetry. One is that triangulating a dome into 20 is the best way of producing a shell of equivalently bonded identical structures. It is the minimum free energy structure." (Cann 2004)
Learn more about this functional adaptation.
Cann, Alan. 2004 October 9. Virus structure. Leicester, UK: University of Leicester.