|Captain Nemo better be thankful he never came across Rapture.|
In-game, the purpose of the Big Daddies is to protect the Little Sisters while the girls cheerfully extract ADAM from the many Splicer corpses littered throughout Rapture. Initially Little Sisters went about this task on their own. But as you can imagine, with such model citizens as the psychopathic, ADAM-addicted Splicers, gathering ADAM was not a walk in the park (or a walk in Arcadia). After suffering numerous Little Sister casualties, Dr. Suchong proposed to create protectors for the brainwashed girls, beginning the era of Big Daddies in Rapture.
A Big Daddy is essentially a Rapture citizen who is spliced to the extreme, giving him extreme muscle mass and agility. The cost is a loss of humanity due to the mind control splices that are part of the conversion process. For reasons not explained, the vocal chords of a Big Daddy are modified to give him a super deep voice which can only make moaning and growling sounds. Lastly, the skin of the subject is removed and the subject is attached directly to a deep sea diving suit, so that the suit essentially becomes his new skin. Due to this being a splicing process that is obviously not very desirable, volunteers were non-existent, leading to forced Big Daddy conversions.
The two types of Big Daddies encountered in Bioshock are the iconic, drill-equipped Bouncers and the rivet gun-equipped Rosies.
As you can see from the following video, these protectors of Little Sisters do their job fairly well; any hostility towards a Little Sister is met with total brutality:
These hulking monstrosities are the most difficult enemies in the game to take down. However, unlike the other enemies in the game, they will not attack you unless provoked, which gives you time to prepare and strategize. Once you initiate a battle (and you must battle at least some of these guys, because you need ADAM from the accompanying Little Sisters), prepare for a thrilling experience as these gigantic tanks try to cut you in half!
This concludes our look at the many freaks of Bioshock, now get out there and mow down some Splicers!