1. Cognitive dissonance reduction: he spent so much, so to avoid cognitive dissonance he's going to tell himself she's worth it. Helps keep marriages together. It is the same reason that hazing ritual happen in college and why secret societies have all sorts of strange initiation rituals, often hazardous or difficult. - Easy way to test this would be RQ: Do couples with relatively more expensive wedding rings for their income level(s) stay together longer than those who spend less on their wedding ring (controlling for other initial ritual costs like the wedding, although I suspect similar effects and interactions there).
2. Symbolic Interactionism This suggests that the initiation rites are there precisely to bind people together socially. It's a social-level theory explanation for all of the rites of passage rituals which are common in mating-couple pairing cultures in humans (and probably other primates, for that matter). A big token like this is a constant reminder of the commitment, and the meaning ascribed to this again helps with the ongoing project. If course this is entirely consistent with the marketing phenomenon, as it's more of an anthropological descriptive approach.
3. and oh... Shiny!