Ok, there can be valid uses. But they haven't happened in my 9 years together with my SO. Probably depends on the habits - I don't call/SMS that much anyway; I know her ATM card code, as we needed that some times, but I've got no idea about her phone/other passwords.
"Your dad sent a text, want me to read it to you while you're washing dishes? I know your mom's in the hospital."
"Mind if I use your phone to call our friends and tell them we're running late? I left mine at home and you're driving."
"Is it OK if I use your phone to put on some music? It's already paired with the reciever."
And so on. Checking a partner's phone doesn't have to include spying. Sometimes it's just the most convenient thing to do.