What is a 2x12 typically used for in residential construction?

Typically used for overhead beams in residential home applications or can be laminated to create a larger dimensional beam for spanning overhead across larger distances when the architect calls for an "open" floor space. Applied with the rise of the beams (12" width) sitting vertically, evenly spaced across the ceiling and braced with cross bracing material and resting atop vertical walls. Creats structural rigidity and suppresses torsional force from both down force (the weight of the force from above i.e. a 2nd floor, roof and/or wind). Build a home with only walls and a roof and push down on it with any amount of force and you'll see, exhibited by the walls, a flex, a twist, then structural failure resulting in a collapse.