If you are programming with windows api, you are more likely to run into the doucment saying:

Reserved. Must be NULL.

If it is always supposed to be NULL, it’s lost the purpose of having that as a parameter there, so why bother to have it there in the first place.

For the purpose of upward compatibility, I’ve already see them making “Ex”, “2″, to name the new version of the same api.

The only “benefit” I see is it’s making the interface not friendly to read.