Possible reasons:

The primary reason enthusiasts package NDS files into 7z archives is space efficiency. While a standard Nintendo DS game might range from 8 megabytes to 512 megabytes, storing hundreds or thousands of these files can quickly overwhelm a hard drive or SD card. The 7z format offers some of the highest compression ratios available, often shrinking the total storage footprint of a ROM library by more than half. This makes downloading, sharing, and archiving games significantly faster and more manageable.

Best for: Desktop users, transferring games to SD cards.