Wilds come in various forms and are an incredibly popular selling point for slots. The main purpose of a wild symbol is to replace many other symbols on the reels (except the scatter symbol in most titles) to help create a winning combination.
The main function of a Wild symbol is to substitute for other game symbols and help make winning combinations. Many slots allow them to perform this task for all except Scatters and other special symbols that can start bonus games. Wilds usually land randomly, but in some games they only appear on certain reels.