Yup, a lot of people don't have realize it, but an engagement ring is a collateral to a commitment: you see no other men because we intend to marry and if I waste your time you're compensated for it with something valuable. Why anyone would use a diamond for something like this is beyond me.

Dowries upon marrying is an extension of this: if I abuse you and you have to leave me you can afford to live your life with this thing I gave you.