WHY IS IT NECESSARY?
Most potable water pipework is made from copper with brass fittings. Freshly installed pipework probably looks clean but is probably not sterile, for a number of reasons. There could be grease and graphite from the extrusion process and bacteria could have got in.
WHAT DO YOU DO?
Flushing new pipework is not always adequate so sterilisation with the appropriate chemical will kill bacteria and also loosen dirt, grease etc.
A building with a water system that has not been used for some time should also have pipework (and tanks) sterilised because undesirable bacteria can easily colonise an unused water system.
Whilst on the subject, everyone with responsibility for a water system should be aware of Legionnaires disease. Showerheads are the most likely source of Legionella infection. Showerheads should be treated regularly with a descaler / biocide. It is no use sterilising showerheads with bleach as bleach will not remove limescale and bacteria can exist under the scale.