It's kind of a misfeature, but the non-evil idea was probably to provide hooks for customizing copy and paste (or other standard command functionality) in benef

It's kind of a misfeature, but the non-evil idea was probably to provide hooks for customizing copy and paste (or other standard command functionality) in beneficial ways, for example seamlessly copying and pasting custom data formats between web apps, or between web and desktop apps. It is a law of the web that any potentially beneficial browser feature will immediately be (mis)used in an abusive, user-hostile manner.