Promise rings

Can someone explain to me the concept of a promise ring? Seems like they're cropping up constantly.

I get the idea, but other than maybe finances (like "we can't get married any time soon because of financial reasons but I promise you my love"), I don't really understand the purpose of a promise ring. Why not get engaged and just have a long engagement? Is a promise ring like "I promise you my love and I'll ask you to marry me one day but not right now?"

Not trying to be an ass, just curious. If you have gotten (or given) a promise ring, what was the reason or motivation? Why not an engagement ring?