The ostensible reason is research on what the range of possible threats are, where you would think that learning how to fight them would be part of that, but is

The ostensible reason is research on what the range of possible threats are, where you would think that learning how to fight them would be part of that, but is instead surprisingly sometimes "not my department" according to Josh Rogin who wrote a book on this. More unofficial and non-scientific reasons that may not necessarily be in play are weapons research, and fetishizing of harmful viruses by sociopathic scientists working alongside other scientists.