There are a couple of ways that this book was successful for Dickens. First of all, it was Dickens's first novel but it was published in installments. Most of the Dickens books we have today were published in serial form in newspapers or magazines. Dickens had been a court reporter and had published several successful sketches under the pen name Boz. Some of these were published as a book in the form of The Pickwick Papers, which was wildly successful.
Little Oliver was also a huge commercial success. He was popular and made a name for Dickens. Although this is one of Dickens strangest and most imperfect novels in a way, it has a storyline and characters so captivating that it is still one of Dickens's most popular books.
Another way the book was successful was in its impact on the social conditions Dickens was trying to bring to the public's attention. The book addresses poverty, the cruelty of the Poor Law, the inadequacy of the justice system and the rampant nature of crime. What an ambitious first book! Dickens managed to do the impossible. He humanized beggars and prostitutes. He got the prudish Victorians to consider the lower classes. Eventually, the book was instrumental in getting the Poor Law revoked.