Letters are pieces of paper with messages written on them intended for one or more people. They may be hand-written or printed out after being typed using a typewriter or word processor on a computer. Before the advent of modern day communications such as telephones, television and the internet, letters were used as the primary form of communication, delivered through either a postal service or a hired messenger. The amount of privacy that communicating through letters affords people when compared to communications over the internet are a big reason why letters continue to be used as the world develops much quicker ways of communicating.
In video games, letters are usually used as devices to further the plot, typically found in locations where the player is not forced to actually see them. Typically letters are included in games to allow players to reveal plot points for themselves if they are actively searching for them while allowing players who don't particularly care about the storyline to pass right by them unhindered.
In other video games, letters play a role as vital communication and the player may be asked to either deliver or intercept the delivery of letters which may have a significant impact on the game's story.
Typically the role of the letter comes down to the setting that the video game takes place in. In stories set in the past the letter takes on a more active role as a useful form of communication, while in stories set in the future or even in the present, the letter takes on the more passive role as more efficient forms of communication exist, such as the telephone and the internet.