Original Wii disc drives are notorious for failing. Once the laser dies, your physical copy of Galaxy 2 becomes a coaster. By installing a WAD, you run the game entirely from internal memory, bypassing the drive entirely.
Disclaimer: Installing corrupt or incorrectly formatted WAD files can result in a "banner brick," rendering the Wii console unusable if proper brick-protection software (like BootMii and Priiloader) is not installed beforehand. Emulation: Dolphin and WAD Files Super Mario Galaxy 2 Wad File
NAND (internal flash) is significantly faster than the Wii’s 6x DVD drive. Installing Super Mario Galaxy 2 as a WAD reduces the lengthy loading screens between galaxies and reduces "pop-in" during the intense gravity flips. Original Wii disc drives are notorious for failing