If the tag is read-only, it can allow:

• marginally better loading, as the printer can heat the nozzle correctly for that filament without input from the user.
• Comparing a G-code file to the loaded filament, either to throw a filament mismatch error, or to adjust temperature settings on the fly.
• Allow slicer software with a network or serial attachment to the printer sense what filament is loaded

If the tag is writable, it can allow for keeping track of how much filament remains on the spool, by writing how much was consumed during each print. This means, when you get to the end of the spool, the printer can warn you if there isn't enough filament remaining without having to manually track the mass of the spool.