What you don't notice until the third or fourth month is the way the shadow gap behind the glass changes with the light. Morning sun hits it flat, but by evening, that little recess throws a thin line of shadow around whatever you've put inside, and suddenly a dried flower or a pressed leaf looks like it's floating rather than pinned. No shop clerk tells you that. They just tell you it fits a five by seven.
Where a box frame falls short is depth, which sounds backward since depth is the whole point, but I mean the opposite problem. Put something too thin in there, a single photograph with no mat, no object, nothing with weight or texture, and you've paid for air. I did this with a stack of christmas cards one year, slid them in flat against the backing, and the whole thing looked like a photocopy somebody forgot to frame properly. The box frame wants a reason to exist, something with actual depth to it, a medal, a dried bouquet, a child's first shoe, a ticket stub next to a photo. Check the depth of the recess before you buy, not just the frame size, because a shallow box barely clears a quarter inch and won't hold anything but paper, while a good deep one will take a small object without you having to jam it in sideways. Check the backing too, whether it's foam board that'll sag in a year or something with a bit of structure to it, and look at how the glass sits, because cheap ones rattle in the frame like a loose window in an old house, and that's a sound you'll hear every time someone shuts the front door. Where this goes wrong is anywhere already busy, a gallery wall full of flat frames where one box frame starts arguing with its neighbors over who gets to cast a shadow. It also looks a bit much in a very modern room, all clean lines and nothing textural, where the box suddenly reads as fussy rather than considered.
Where it earns a spot is the ordinary hallway, the one with coat hooks and a bench nobody sits on, or a bedroom dresser top next to a lamp and a dish for loose change. It sits well with worn wood, with linen, with anything that already has some texture to it, because the box frame is basically asking the rest of the room to match its seriousness.
It gives a flat wall somewhere to put its weight down.