A megohmmeter (shortened to "megger") measures the resistance from motor windings or electrical cables to ground, or earth, or the metallic enclosure surrounding such windings or cables. A low resistance means that the insulation is compromised, whether from moisture in the insulation, tears or cracks (breaches) in the insulation, stray strands of wire, etc. Such conditions can be hazardous, hence the importance of testing for them.

It confirms that the installation is in sound condition before mains power is applied to it for real. Were there to be a fault, then the circuit protective devices would operate repeatedly until the fault were cleared. Using a high voltage insulation tester beforehand establishes a high level of confidence that the installation will operate correctly and safely when power is applied in earnest.

The megger is used to pass a surge of high voltage (selected based on the voltage rating of the system) and in this stressed condition checks for insulation resistance. Unlike the normal say multimeter where one will check the resistance in unstressed condition.

Thus by Megger, you are confirming the actual status.

Just a reference, but quite a good one

Just to ensure that the insulation is capable of withstanding the voltage that is to be applied across it during its normal operation.