The Office had its share of quirky, offbeat characters, but there was no character as mysterious, creepy and downright demented as Creed Bratton. A comedic rendering of the eponymous musician of The Grass Roots fame, Creed was prone to random outbursts, cluelessness and bizarre, often disturbing revelations about his past. After the documentary depicting the Dunder Mifflin workers aired and his criminal activities were revealed, Creed faked his death in the warehouse bailer and disappeared, only to resurface once again in the series finale, revealing that he has been hiding out in the office.
As the show put little focus on Creed's work duties as opposed to his strangeness, many (including occasionally Creed himself) do not actually know what he does in the office. However, he does have an important role in the Scranton Branch, though he often has to fight to keep his job.
Despite rarely being seen doing any actual work, Creed is established as the Quality Assurance Manager at Dunder Mifflin's Scranton branch. Essentially, he makes sure that the quality of Dunder Mifflin's products are up to par. As part of his duties, he is supposed to take part in a weekly spot check at the local paper mill that supplies their paper stock. However, Creed is terrible at his job, and his full negligence is on display during the Season 3 episode "Product Recall." In the episode, a NSFW watermark is on all of Dunder Mifflin's recent paper shipments. Creed reveals in a confessional that he had been blowing off the spot checks that would have prevented the disaster. Creed doesn't just ignore his job duties -- he doesn't even seem to understand what his job is. In "Goodbye, Toby," Holly Flax asks Creed what his job is, and he cannot even remember his title.
Creed also has other side businesses outside of his Quality Assurance gig. He takes part in various criminal enterprises, ranging from selling fake IDs to students, to constant theft, and there are even implications that he sells drugs and possibly even murders people. In the series finale, it is finally revealed that during his time in The Grass Roots, he "sold drugs and trafficked in endangered species meat and stole weapons-grade LSD from the military." His eventual crimes catch up to him, and he is arrested by police in the closing minutes of the finale.
Though Creed's job seems inconsequential, he also seems to desperately need the job. He makes many references to him being homeless and even sleeps in the office a few nights a week. He also goes to great lengths to keep his job, going so far as to convince Michael to lay off another employee instead of him and framing a paper mill employee after the "Product Recall" incident.
Perhaps he is so desperate to keep the job due to his desire to stay off the streets (or to fund his scuba dreams), but his deranged nature seems to take over his other priorities. His job performance reflects this and is probably why many people do not actually know what Creed contributes to the office.