Looking back Loitering is not in the 1991 laws - so do we, nonchalantly looking at OB, know why it was added?
Check again.[LAWS]1991 Law book
Law 25 (3) NOTE (ii)
The referee should be careful to ensure that no benefit under Law 25 (2) is gained by loiterers who wilfully remain in an offside position and thereby prevent opponents from running with, kicking, passing or otherwise playing the ball.[/LAWS]
I can also tell you there was a significant change in 1992. Before then it was legal to chase a kick even if you were in front of the kicker, as long as you stopped at least 10 metres before reaching a player waiting to catch the ball (etc).
I have been looking into the history and it is difficult because
(a) before 2000 there were separate laws for offside and onside, and offside covered all phases of play. Currently Law 11 really only covers General Play.
(b) Many concepts have changed. The first mention of "loitering" I have is 1959, when the ruck was a "loose scrum", wing forwards could detach early etc. Mauls were not mentioned.
This makes it hard to be sure quite what was intended at various times (and there is no guarantee that views remained consistent over the years as the game changed). I might try to put together a historical perspective, but that does not necessarily help in today's world.
Laws 11.3 and 11.9 seem to be contradictory. If a player is put onside by 11.3, then he is no longer a loiterer, but 11.9 still wants to inhibit his actions. He can't be put onside by a team mate, since he is already onside. 11.8 covers most of what was covered by the pre-2000 laws and 11.9 may be intended just as a catch-all.
As I say so often, we should not be trying to apply logic to the wording of the laws, but to the best interests of the game.