Everything you always wanted to know about how Nintendo cartridges work* you can learn at nesdev.com. Like the proper method for cleaning: rubbing alcohol on a cotton swab, NOT blowing.
In-cartridge save slots are great until you want more slots than the cartridge provides. Generally that's either one (autosave) or three.
Three is fine when each is just a player profile (how many people play your NDS, anyway?), but it's not always enough for things like custom creations (e.g. map editors), or checkpoints in a campaign where you could screw something up and have to backtrack. And games with only a single autosave slot (e.g. Nintendogs, or any game with global history tracking like Advance Wars DS) means one person per cartridge, period.
Of course, like using an ARMAX to copy PS2 saves to/from a USB memory stick, I believe there are third-party utilities to do this for NDS carts. But while on-cart save slots are way convenient, they do have their drawbacks in terms of flexibility.
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Anonymous2009-03-18 3:04
I'm feeling really keen, for some of that good ol' green