Initial verification and testing is mainly intended to rule out installation issues such as faulty connections or pinched wires, to some extent also manufacturing defects (it's rare but cables sometimes do have non-continuous conductors in the middle of a roll, just to give an example). It's typically performed by the contractor who did the work. Commercial premises are also required to be tested periodically, intervals depending on the expected stress on the electrical system. Harsh environments require testing every 6 months in Austria while offices are only to be tested every five years. Switzerland even includes any homes, which have to be tested every ten years, regardless of occupancy status.

Socket testers show that all conductors are connected to the right terminals but they won't usually flag poor (high-impedance) connections. So most regs require low-ohm testing of any protective or bonding conductors before an installation is energised. In periodic inspections, the Rlow test can usually be substituted by a loop impedance (Zs) test.

Ring final circuits obviously make even more continuity testing necessary.

In many countries, owning a multi-function tester (insulation resistance at 500 VDC, Rlow, Zs, PFC/PSC, RCD test) or individual testers covering all the above, is a requirement for operating as an electrical contractor.