NYC Robert: Relationships and the phone, should your significant other know your password?

Today's topic came to me after listening to the radio late one night. The host was asking callers what they think about their significant other having the password to their cell phone. Before she took a caller I thought to myself that I wouldn't have a problem with my significant other having the password to my phone. But I would have a problem with her going through my phone without asking. Some of you may say, "well what do you have to hide?" And I would say, "I have nothing to hide. It’s just that I feel if my partner trusts me why is there a need to go through my phone?" So my question for those of you who feel you should know your partner’s password is: why? What is the purpose of you having it? Now, I'm not saying I wouldn't give her the password, but what puzzles me is if she has her own phone how come she needs mine?