That's where she keeps her BJ videos. That's where she keeps her nudes and lewds. Its where all of her weirdest and nastiest convos are kept. It's where she stores the info of all the men she's ****ed, some of them she wants to keep secret out of shame or because they're married/committed. Everything she won't tell you, her phone's info will.
Nowadays women store their entire life on their phone, and guard it as vehemently that a weaboo would guard his hentai stash.
I remember this one girl I worked with left her phone unlocked and on the table to take an unwarranted 25 min break, and I spent my time just scrolling through her stuff. While she did have a boyfriend and praised loyalty, played the good christian girl, found out a lot just from a few nosy button presses. Slinging nudes, talking about wanting to be ****ed without her bf knowing, dressing up in the bathroom for guys (like snapping pictures of her slutty lingerie in the mirror I cleaned), and getting utterly depraved in the chat. Apparently she dumps the kids off on him, goes to visit her "mom", but not really, she's doing quickies in another guy's truck.
When you get ahold of a girl's phone you learn EVERYTHING she doesn't tell you. Many more experience girls know to lock their phones, and delete conversations (or use snapchat so their convos are auto-deleted), so yeah. If you ask to look at your GF's phone, and they hesitate, its guaranteed that they have something they don't want you to learn. Otherwise it would've been filled with convos of her just saying no and i have a bf.