I don’t understand promise rings???

Y’all don’t hate on me but I just don’t get promise rings. Like just get engaged? Why spend money on two rings? Why do you need to promise to ask someone to marry you? That just seems like a terrible and high school thing to do. I mean you do you, but I just know I’d never ask my SO to give me one or I would never give my SO a promise ring. I trust him he trusts me and knows I’m committed to him. Anyone care to explain? I mean I definitely am not gonna change my mind but maybe explain to me why they’re a thing?