Insightful breakdown of Butt plugs:

butt plug tail
my bf found a pic of a butt plug that had a bushy animal tail attached to it so it looked like you had a tail when you put it in. he thought it was..

Which one of these plugs would you choose?
Which one of these plugs would you choose?

The Snooze Button
When you set your alarm clock, do you set it early knowing that you can hit the snooze button for some extra sleep? Or do you set it precisely at a...

What's the best plug shape?
It seems companies are making more and more creative shapes, but I still prefer the classic tapered cone. Whats your favorite?

Mood naughty medium plug
when will they ever restock this amazing product. I wanna try it so bad.

They can be used to simulate dual penetration, provide a full feeling, extra stimulation during sex or masturbation and some can massage the prostate.

In addition to the reasons stated in the two previous responses, some people definitely use butt plugs as a tool in D/s relationship. This can be done as a disciplined training ritual, punishment, and even as a symbolic reminder. For instance, my Mistress decided that some butt plug training was in order early on in the start of our 24/7 relationship.
Forced/required long-term plug wearing is a quite effective means of correcting unacceptable behavior in my experience. Any moodiness or disrespectful behavior is difficult to keep up once you realize that someone has taken total control of such a sensitive area of the body, including its biological functioning. The turnaround on the plugged up submissive's bad behavior is impressively fast.
Lastly, butt plugs are pretty common amongst kinky people perhaps because surrendering one's body and ultimately having your ass stuffed and made off-limits is a humbling reminder of one's subservient status with respect to his/her Master/Mistress. Consider one's ass to be the "lowest common denominator" of the whole body... if someone else controls that part of you, its doubtful you have power over anything else for that matter...
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Butt plugs...stimulation and arousal, provides a stuffed/filled sensation, massages the prostate, anal sex preparation tool; and last but not least, one pretty darn complex psychological control device... ;D