Det mycket korta svaret är att Sarlacc får energi från att smälta icke-känsliga varelser (womp råttor, eopier osv.) och sedan använder den energin för att tortera sina känsliga offer, att få ett mått på telepatisk njutning från deras plåga.
Faktum är att det gillar deras smärta så mycket att det förbrukar energi för att hålla dem levande så att de kan hålla tortur på dem nästan obestämt.
The mature Sarlacc, however, does have mobile tentacles and legs but has adapted its legs as anchor roots. Scientists currently believe the Sarlacc is an animal, much like sponges and anemones are animals. Because it lives in the middle of the desert, the Sarlacc does not feed often, but because of its highly efficient digestive system, it doesn't need to. Its body preserves food for incredibly long periods of time, digesting it slowly and storing it until the Sarlacc needs nourishment. Unfortunately, the victim often remains alive for much of the time, in part sustained by the Sarlacc's internal nutrients.
One of the prevailing rumors about the Sarlacc is that the creature is mildly telepathic and actually gains knowledge and sentience from victims as it consumes them, sometimes over thousands of years, depending on the species of the meal. Some data Senior Anthropologist Hoole secured from the bounty hunter Boba Fett have confirmed this.
Fett's helmet recorder was running, apparently, during a period in which he was trapped inside the creature. When I sat down to study the tape, I was horrified. Not only was it clear to me that the Sarlacc was sentient, but it enjoyed torturing those it was digesting. Fett's actions and responses plainly indicated that the creature manipulated the thoughts of its victims, and even kept their intelligence stored in its memories so it could savor their pain at another time.
The recordings also showed a more anemonelike physical structure than most scientists have believed, and the secretion of some digestive enzyme that might be the cause of their hallucinogenic power over their victims. This theory was supported by the fact that Fett could plainly be seen reacting to stimuli that were not there.