Mobile Golf for GBC is a sequel to Mario Golf with heavy emphasis on online play. While there is a good single player mode, some contents can only be unlocked while being online. Since the online service was shut down in 2002, the content is unavailable today. This save unlocks this content.
Snow-Land short course
Rowin-Night short couse
(Keep in mind that those courses are not available in all modes)
Mario Golf characters (Kid, Grace, Tiny, Gene)
Mario characters (Mario, Peach, Yoshi, Foreman Spike)
Golf clubs with different stats. Available through "Character" (bottom right) on the main menu.
Save file contains a character on save spot 1. It might be safe to delete it, but I haven't tested if the content will still be unlocked afterwards. If not, you can still start a new game on spot 2 or 3.
Additional information for nerds:
All the content is already on the cartridge and was unlocked with the following cheat codes via emulator:
If you want, you can try to apply those codes on your own save file!