Writing letters to ppl in jail is a quick and easy way to make their jail experience suck considerably less than it otherwise would. Listen: Reminding people that they're still supported and thought of is great and v. important, but in a way it's also a demonstration of force. Being in jail means being seen as unpersoned, disposable, and expendable by much of the rest of society and by one's captors and their collaborators especially. Someone who gets an asston of mail is more likely to be seen as someone whose disappearance or mistreatment might have professional consequences.