Introverted tweens may particularly benefit socially from having a pen pal. A pen pal gives them the time and space to find the right words (they often freeze if they have to talk before they think) and naturally promotes a deep, less superficial relationship that introverts crave.
Sending a letter is a very personal thing, You can create a special and unique parcel for Your recipient. Collecting pretty papers to have on hand to write whenever You want to is the ideal. Not too many to get in the way unless You have a huge office and filing cabinets to store it correctly (like Me the sucker!)
Receiving a letter on beautiful stationery is lovely, with Someone Who has nice handwriting and has taken the time to write a long, detailed letter.