Games don't exist in a vacuum anymore.  Sometimes a game knows that you've played other games.  Better yet, sometimes players are rewarded for such behavior.  There's a discussion at the NeoGAF forums involving games that give players bonuses or upgrades based on memory card data from other games.  For instance, the main Ratchet and Clank games for the Sony PlayStation 2 read memory card data from the previous game in the series.  If you have the right data on your card then you get free or discounted weapons.  Then there's the more obvious games that use this gimmick, such as Sonic & Knuckles's lauded lock-on abilities from the Sega Genesis era.

Personally, I like these little bonuses provided it's possible to access the content in other ways if I haven't played other games in the series.  When I played the Ratchet trilogy on PS2, I played them out of order, starting with the first game, then the third, and finishing with the second.  By doing that I missed out on some freebies, but I was able to earn those freebies with hard work and a large collection of bolts (read: in-game money).  Sometimes formats get in the way.  For example, Mega Man X8 for the PS2 features a secret fan service battle with an 8-bit Cut Man all the way from the original Mega Man.  This battle only happens if there's a save file for Mega Man X: Command Mission on the memory card.  Alas, I have the Nintendo GameCube version of Command Mission, and of course PS2 and GameCube memory cards aren't exactly on speaking terms.  So was I left out of the Cut Man fun?  Nope.  Capcom tossed in a secret code to unlock the battle without having the required save file.  No harm done.