perception
Common region
Things sharing an enclosed area – a card, a bordered box, a background block – are perceived as a group, even more strongly than proximity alone.
Draw a box around some elements and the brain files them as one unit – it will even override distance to do so. This is why “put it in a card” is such a reliable design instinct: the enclosure isn’t decoration, it’s doing real cognitive work.
It’s also why cards are so easy to overuse. Every border you draw is a grouping claim; boxes inside boxes inside boxes make the page’s structure louder but not clearer. High-craft interfaces often need fewer enclosures than you’d expect, because good proximity is already doing the grouping silently.