Well, first of all, that really depends on the context of the promise offered along with the promise ring! Most commonly, a promise ring if exchanged between lovers in a romantic relationship promising to love and commit to each other, of course, this is very similar to what a couple getting engaged will commit to.
But the commitment that comes with an engagement ring, the commitment of wedding is much more formal and long lasting than the pledge of a promise ring. Usually, promise ring will be given when the couple is not ready yet to get engaged for whatever reason. It could be due to financial considerations, the early phase within the relationship or even when examining when promise rings are given you’d see it often simply a matter of age.