What does it mean when women call their partner their best friend?

I see it a fair bit in general life but also at weddings (bride to the groom in her speech).

I assume that it signifies a change in the relationship from dating, where she does whatever he wants in order to catch him, to now he is caught and she can have her way with him, and he provides etc...

It is a bit reminiscent of Golem's, "My Precious."