Unfilled gaps and cracks in the foundation, around windows and doors, vents, and so on, may let cold air in the same as leaving a window open.
In fact, a 1/8 inch opening around just two doorframes can let in as much cold air as a 12 inch window opened 6 inches all winter long.
Caulk is used around outside window and door frames,and to fill outside wall and foundation cracks.
The money you spend on caulking or weather stripping is usually recovered in one heating season or less.This one season "pay-back" period means that money for heating fuel is saved equal to or greater than what you spend for caulking and weather stripping materials.
Some but not all the places you should look when surveying you home before caulking are around doors and windows, dryer vents, faucet pipes and wires, where porches attach to the house, seams between masonry and siding, chimneys and inside corners