I can imagine lots of reasons to use my partner's phone.

"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.