You've probably heard it said that you should shut off your hose bibs for the winter season. When the weather dips below freezing, any unprotected water pipes, such as those leading to a hose bib, are susceptible to freezing. When water freezes it expands, and can exert a pressure of up to 2,000 pounds per square inch. That's a lot of pressure in a solid pipe with nowhere to go, leading to bursts and costly repairs.
Most water pipes within a home are okay during the winter months because our homes are heated and insulated. It's the water pipes in external walls, crawl spaces, and other unprotected areas that are most susceptible to damage.
Consider that if a pipe bursts in a home, there's a heavy flow of water leaking out in spaces that aren't always easily accessible (usually in the walls). Shutting off the water valve quickly can stem the problem temporarily, but there's a lot of damage that can occur beyond the burst pipe, such as wet framing, flooring, drywall, and more. Even if there isn't much immediate damage, a plumber will need to first find and then repair the broken pipe, which may be behind drywall (or worse) that will need to be broken into and cleaned up afterwards. All of this can cost quite a pretty penny, which is all the more incentive to shut off those hose bibs in the winter.
Remember after shutting off water to a hose bib to also open up the outside faucet and "bleed" out any remaining water left inside. For a typical hose bib, the water pipes leading up to it are always full and under pressure (how much pressure depends on variety of factors such as the supplier, pipe size, demand, etc.). Practically speaking, that also means that water is fed from the source all the way to the hose bib itself at all times, and while shutting off the valve leading to the hose bib may cut off its future water supply, the pipe is still full of water which can freeze and cause the pipe to burst. So be sure to let out any water leftover leading up to the outside faucet. Shutting off the supply is only half the battle.
Installing a frost-free faucet (also known as a sillcock) can help reduce the necessity of turning off a hose bib in more temperate climates, but these devices are not 100% freeze proof. They provide a layer of protection by keeping the water away from the freezing air when the faucet is not in use.