Unlike the Klingons of the original Star Trek, who were hostile to the Federation of Planets (which includes Earth), Worf and his race are allies with the Federation.
Star Trek lore has it that the show’s producer, Gene Roddenberry, at first didn’t want Worf as regular in the series. Apparently Roddenberry didn’t want to rehash existing themes (like the standard alien race of “Klingons”) from the original TV series.
But Worf’s popularity was undeniable, and Roddenberry, along with subsequent producers, changed their outlook on the Star Trek mythos. Instead of avoiding past episodes, they embraced and adapted them to enhance the overall story, which is a literary device called ‘retroactive continuity.’