builders often don't, saying that heat lost from pipes within the fabric of the house just helps to warm it.
The fact that they can't be bothered is seldom mentioned.
Pipes under ground floors, or in unheated parts of the house, especially lofts, or on an external wall, should be insulated. Not only will you be wasting heat, you may get a frozen and burst pipe. An extra-thick grade of insulation, called "Water Regs" or "BS" is much thicker than cheap stuff.
Hot pipes around a boiler or cylinder, and going to hot taps, should be insulated.