The Bocchicchios were important because they provided an important service to the other families. But they weren't one of the Five Families. The novel provides the needed detail. Puzo wrote that the Bocchicchios dominated a local garbage collection racket. But they were basically small-time. "A streak of stupidity ran through them," he wrote. Also they were "straight from the shoulder people who knew how to bribe a policeman but were unable to approach a political bagman" (I'm approximating those quotes).

SPOILER:

The hostage and negotiating business they created was based on the fact that, if the representative of the side that hired the Bocchicchio hostage were killed by the other side, the victim's Family would kill the hostage. Then the Bocchicchios would take their revenge on the other side. They were so simple-minded that nothing would deter them. "A Bocchicchio hostage was gilt-edged insurace," said Puzo. But the hostage business is hardly the stuff of one of the Five Families.
The story of how the Bocchicchios helped Vito to bring Michael back from Sicily is, IMO, one of the best, if not the best, backstories in the novel that never got into the movie.