Its a christian culture thing mostly however some people do take part in it outside of christianity. It can be a promise in multiple ways depending on the relationship. I.e a promise ring typically means your promising to wait till marriage to have sex and you promise to be “pure” and also not cheat.
As a christian myself i didnt use one. Ive made some mistakes in my relationship from a christian viewpoint (sex outside of marriage). I love my gf very much and we both talk about how we wish we had waited but idk its a lot harder then people say expecially when your long distance.
The hard part tho for like my case is when me and my gf started dating we had 4 years. We are now 2 years into dating. Weve both been commited and new we wanted to get married sine before we dated we just waited to get out of highschool. Im not sure why people propose then dont get married quick. I understand that weddings are expensive but you can legally marry someone and start figuring life out before you hold the cerimony