I suspect that Charles Dickens invented Orlick because he intended to use him in a future installment to attempt to murder Pip's sister. This would lead to Biddy's coming to live and work as housekeeper and nurse in the Gargary household, and it would eventually lead to Joe marrying Biddy when Mrs. Joe finally died as a result of the injuries inflicted by Orlick. The assailant remains a mystery for a long time. Then in Chapter 53, when he has lured Pip to an isolated rendezvous and is preparing to kill him, Orlick reveals the truth.
“I come upon her from behind, as I come upon you to-night. I giv' it her! I left her for dead, and if there had been a lime-kiln as nigh her as there is now nigh you, she shouldn't have come to life again."
Apparently Mrs. Joe never knew the identity of the man who had nearly killed her, since she had been attacked from behind. She was unable to speak for the remainder of her life, so she could not provide any information that would have been useful in deducing the killer's identity. Orlick was naturally suspected, but he seemed to have an alibi. He has no qualms about telling Pip how he nearly killed Pip's sister, since he expects his helpless victim to be dead in a matter of minutes.
Great Expectations was published in serial form in Charles Dickens's weekly periodical All the Year Round between December 1, 1860 and August 1861. Dickens was writing the novel as it was being published, and he probably did not know exactly how he would finally expose Orlick as the man who had nearly killed Joe's wife. But Dickens was a genius, and he undoubtedly knew that he would come up with some good scenes when the appropriate time arrived. In the meantime he could let his readers wait and wonder. It was questions such as this that kept Dickens's readers on both side of the Atlantic waiting breathlessly for the next installment.