The most basic function of a keychain is to hold keys. Although there are many keychains that have their own specific features, such as bottle opener keychain, carabiner keychain, decoration keychain, flashlight keychain, etc., it depends on how people want to use it.  
Given the cost, the keychain is an ideal item for any promotional or advertising purpose. You can put your brand, slogan, logo or image etc. Increase public exposure on the keychains and then realize its commercial value.
Besides, keychains make great gifts for many occasions and holidays. They can be designed according to different themes, such as Valentine's Day, wedding anniversary, birthday, etc., and make a good souvenir gift.