Mothers' Union members make cards and send them to people who cannot get to Church or have moved away. Sometimes, cards are sent to people who members know are very busy, who feel they don't have time to come to Church.
Writing cards enables members to let those people not at Church know that Mothers' Union and the Church community still care and think about them. Those receiving the cards will still feel part of the Church community even if they can't be there.
(Please note that you will need to check first with the intended recipients that they will be happy receiving such correspondence.)