Ever had to use a d-pad to type out a password that looks like this? "000ys T0J300 m2001C H000aS" (Kid Icarus).
Ever had to wonder if that was a 0 or an O... or a number 1, capital I, or lowercase l?
Ever tried to play a game where they got "cute" and used pictures or symbols for their level passwords, instead of simple letters? (Microcosm)
Then thank battery backup!
Battery backup is the term for storing a player's progress within a small battery within a game's cartridge. The immediate benefit is that players can instantly resume progress from where they left off without inputting a memorized password. The downside however is that the save is not perminant. While the battery lasts a long time, a long time isn't forever. It's estimated that most of the batteries used to perform this function had a lifetime of about 10 years after which a players save would be corrupted. To use an example, if you were to of played Metroid when it first came out and jotted down your password, you could still play that file today. If you played Super Metroid at launch, however, your save file probably doesn't exist anymore.