The practice may be pleasurable because of the large number of nerve endings in the anal area, and because of the added stimulation gained from stretching the anal sphincter muscles while inserting the finger.
A good quality personal lubricant is advisable to both increase the pleasurable sensation and aid insertion. Some people prefer to simply stimulate the outer ring of the anus, while others will follow this by inserting one or more fingers. Fingering may be seen as an act in itself, or as an arousing prelude in preparation for further anal sex. Anal fingering can arouse the receiver, allowing them to relax their anus and prepare them for the insertion of a penis or any other sexual instrument.
Anal fingering is also an effective way of stimulating the prostate in males, and thus may bring the receiver to orgasm. Anal fingering can also stimulate the perineal sponge in females.