Here's a direct quotation from the community's profile on Dreamwidth:
The mission of the Multi-Fandom Postcard Exchange is to allow fans to anonymously send and receive postcards using us as an intermediary because we believe that small acts of kindness can have life-changing effects.
We aim to be no-stress: you find people with similar interests and write to as many or as few of those people as you’d like, as many times as you’d like, whenever you’d like. And to ensure that money is not an issue, you can join without sending a single postcard. There is no pressure to perform.
We especially want to reach out to lurkers. We recognize that lurkers often make up an enormous part of fandom and lurk for a variety of reasons. We welcome all lurkers to join and lurk about our member directory. Should something inspire you to un-lurk for a moment, you can do that and then go back to hiding in the void. We don’t mind at all!
We also recognize that many people may be experiencing (sometimes greater) feelings of isolation during the pandemic. We think our postcard exchange is useful because 1) it allows for low-stress interaction and 2) it gives us something physical to hold onto/squee over/smile about.