The most basic function of keychains is to hold the keys. While there also many keychains have their own specific functions such as bottle opener keychains, custom bottle opener keychain no minimum, carabiner keychains, flashlight keychains, etc..It depends on how people want it to be used.
Taking costs into consideration, keychains are the ideal items for any promotion or advertising purpose. You can put your brand, slogan, logo or images, etc.. on the keychains to improve the exposure to public then realize it commercial value.
Keychains are good for being gifts in many occasions or festivals. They can be designed according to different themes like Valentine's Day, wedding anniversary, birthday, etc..It is a great gift as a souvenir.
Keychains can be fashion decorations to your dress up.