What is the point of karma then ( which is another and perfectly valid question -- why not just get rid of it... ), and why it grants benefits as it goes up when it is ( seemingly ) given entirely at random ?

To get karma, you need to have had some level of participation in the community (posting useful articles or making useful comments).

By putting a karma threshold on actions like flagging or downvoting, HN ensures that these actions can't be done by someone who has just created a throwaway account to harass people.