Promise rings are commonly purchased as a promise to love the one you're with and to not date anyone else as long as you are with that special someone. Promise rings also can be worn or purchased for other reasons.
A promise ring can be purchased and given to someone as a message that your loved one will be returning to you. A man or woman going to fight in a war may give a promise ring to their significant other as a promise to return.
A promise ring may mean that your partner wants to marry you but just isn't quite ready yet. It may mean that your partner pledges to save himself or herself for you until marriage.
A promise ring may also be purchased and given to your loved one with a message from you. The message here could mean that you make a promise to yourself and your partner to stop drinking, gambling, smoking, or whatever vice you may have that is damaging your relationship.