In the olden days of Earth, before communication devices were found in everyone's hand at all times, humans wrote letters to each other as a way of keeping in touch. Communication in those days was slow, taking days to weeks to get from the sender to their intended recipient based on the distance between them and how efficient the mail system was at that time.
For example, within a major city like London in the late 1800's, letters were often delivered within hours of being sent. In an odd twist, as time went on, delivery times within a city like London slowed down with one to two days delivery time being common. Eventually people switched to using Electronic Mail (email) or telephone messaging (sms) in order to get their messages through faster.
The longest mail delivery appears to have been for a letter that became lost in a post office that was found and delivered 89 years after it had been sent.
Letter writing was an art form. It involved careful choices being made from the paper to the ink, and even to the images used on the stamps. At the height of the letter writing period, people took special classes (called penmanship) to learn to craft their letters so that they were both beautiful and legible.1 Special paper and envelope combinations were made to convey different moods and impressions. These sets were known as stationary.
Letters were used for everyday communications and for business. Customized scams or chain letters were created to take advantage of the unsuspecting or the bored. There was also a special style for love letters, or the dreaded "Dear John" breakup letter. This communication art form created mementos that were kept and saved for future generations, often revealing a person's deepest secrets to their ancestors many generations later.
Security was a huge issue with letters as anyone along the letter's path could steam the envelope open and read the contents. Some individuals wrote their letters in code to try and help protect their privacy. Many governments routinely inspected the mail of their citizens using both manual and technological means.
While still practiced on Earth, letter writing is considered quaint and obsolete unless the letter is being used to commemorate a rite of passage like a graduation or wedding. Receiving a hand-written letter is considered a sign of particular affection.
Those who wish to enjoy this very human art form can ask the concierge for stationary and a pen. Stamps are available for purchase and there is a letter drop at the front desk. Be aware that letters can not be sent off-planet, so you'll need the Earth address for whoever you wish to contact. Sending letters to unknown recipients is considered quite rude.
1Note that despite this attention to detail, there was another belief that the most intelligent among the population would write with the least amount of legibility giving rise to an entire communication form known as doctors' scribble.