There are a huge number of factors, big and small, that affect the overall amount of goals scored within a league season -- number of powerplays, talent of goalies, talent of shooters, evolutions in technology (curved sticks! bigger goalie pads!), evolutions in training, evolutions in tactics, literal number of games played, and so on. The game has changed in many ways throughout it's long history such some sustained stretches of seasons have had almost 80% more goals scored in them than others.
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Adjusted points are a a way to approximate production relative to the scoring environment that a player played in. Everyone intuitively understands that a goal was harder to score in the dead-puck era than the late 80s. Adjusted goals are an attempt to quantify how much harder it was. For the actual calculation, I'll just post the HRef link as they do a very good job explaining it.
Explaination of hockey stats adjusted for the era the games were played in.
www.hockey-reference.com
As for the effect that the all-time greats like Gretzky and Lemieux had on leaguewide scoring environments -- in 84-85 there were 6,531 total goals scored in the regular season. Gretzky's league leading 72 goals were barely over 1% of the total amount scored that year. If you are even more generous and imagine that had Gretzky never existed none of the goals he had a point on would've been scored (a very flawed assumption IMO), that is still only a decrease of 3%. Scoring dropped 34% from 84-85 to 03-04. The disappearance of Rushmore-type players in their prime is only a small part of the much larger story of scoring changes.
Overall I think they're a very important tool when comparing the counting stats of players across eras.