Letter writing is relaxing, calms the brain, but also allows me to think about the person I am writing to, and craft measured replies. I'm pretty sure at some point in our lives, we've all opened our mouths and said something without thinking. I know I have done and could well have ruined friendships if I hadn't realised straight away what I had said. Letters also allow me to communicate with someone while they are otherwise busy (work, chores, enjoying entertainment or socialising) or are asleep. Sending email would also allow for this but the ping (or other sound notification) on a mobile phone can be distracting at inconvenient moments. I found email social correspondence too demanding. "I know you have read my email, why haven't you replied?" was one follow up after not replying the same day.
I write or start a lot of letters out in public, in cafes. I even take with me some washi tape and a means to cut it to size.
Although I have more than enough stationery to last a lifetime, I do look for more stationery. I couldn't resist this set, Emily's Cats originally sold at WHSmith, found in a charity shop for £1. The box had never been opened.