The older you are, the more convenient and comfortable staying anonymous becomes, especially if you have or have had serious responsibilities over the years. You are either a target and/or intimidating, and either one gets in the way of sharing information freely.<p>The older you are, the less interest or value there is in maintaining an online persona, especially in a community much younger than you. But the insights from such discussions, and the ideas you can provide, can still be valuable.<p>Hence a former Secretary of the Air Force becomes DiscoDuck, for example.
I think it is in part because after a time limit comments cannot be deleted. Someone may not want to permanently be associated with certain comments if they shared other details in other comments that would identify them
A lot of the stuff discussed on HN is specific to fairly small communities so if someone wants to keep their anonymity they may need to use a throwaway account when sharing information on a particular topic.
HN is not immune to the shill accounts, similar to what you see over on Reddit. You see these accounts come out of the woodwork on political posts, but they engage elsewhere to establish a history.