Because if they freeze they may burst, then you will have a mess and no water.
It is important to protect water pipes so they don't freeze because when water freezes, it swells. If water is trapped inside the pipes the pressure from it swelling will cause the pipes to burst.
When the temperature is 47 degrees, NOTHING is in danger of freezing !
Water EXPANDS as it freezes, hello.
Result is broken pipes, so symptoms are: 1) no Water at faucet 2) leaking water elsewhere.
Heat tape is just for keeping the pipes from freezing, it does not heat the water. That's what the hot water heater is for.
There is no single best way to keep pipes from freezing. Insulating (wrapping) pipes is a common method. Leaving the cold water faucet open just enough for a slow drip is another method to prevent freezing.
Assuming you don't mean water heating systems, and just mean normal fresh (drinking) water systems, then they freeze at 0 degrees Centigrade like all water. But in the case of for example a home, the plumbing protected from freezing in a few different ways. Firstly most homes that are lived in have some form of heating, so this keeps the ambient temperature of the water in the pipes above freezing. Pipes that are in places more likely to be exposed to freezing, such as lofts, and roof cavities will be insulated with pipe lagging to protect them from freezing. Plus remember those pipes in roof cavities will benefit from heat rising from the home/building, helping to keep those pipes above freezing. If the subject of your question is in relation to 'burst' pipes caused by frozen water in the pipes expanding and literally tearing the pipes apart, then this will start to happen at about 20 Degrees Fahrenheit or -6 degrees Centigrade. Remember that a little freezing is not a major problem, as plumbers sometimes use special freezing kits, that will freeze a short section of pipe to block it off temporarily, while they work on the pipes.
Yes, they can
water expands when frozen.
If you don't insulate water pipes or protect them in some way from the cold, the water inside the pipes can freeze. This causes you to be without water until the pipes warm up enough to thaw. Also, when the water freezes, it can expand and burst the pipes which will then need to be replaced. Save yourself the aggravation of having no water and the cost of replacing pipes by insulating them. To protect pipes in an empty or unused building, you can pour anti-freeze down all drains and in the toilet to winterize them.
One can prevent pipes from freezing during cold temperatures by ensuring water runs through them. A good idea to do this is to let the tap drip. Draining the water system when going on vacation during cold spells can help pipes from freezing as well.
Pressure has nothing to do with pipes freezing. It is just exposure to cold.