[1] It shares the C-terminal regulatory domain (InterPro: IPR019523) with protein phosphatase 1 subunit 15A/B.
EIF-2A is required for translation so by shutting down eIF-2A, the cell prevents the virus from hijacking its own protein-making machinery.
Viruses in turn evolved ICP34.5 to defeat the defense; it activates protein phosphatase-1A which dephosphorylates eIF-2A, allowing translation to occur again.
A herpesvirus lacking the γ34.5 gene will not be able to replicate in normal cells because it cannot make proteins.
[1] The ICP34.5 deletion is useful for the construction of oncolytic herpes viruses, as cancer cells do not restrict replication as strongly.