Mail may be the most confidential way to communicate.
Got a secret? Still in the closet? Secret lover? Crisis of faith? Abusive situation? Got something you're embarrassed to say? Afraid of censorship? Wanna plan a suprise party? Just don't want anyone to overhear?
You can write whatever you want!
Ring doorbells, cell phones, and nanny cams can be used to record you. Phone calls can be tapped. Texts can be accessed through your cell phone company. Emails can be hacked. And any smart device you own with a microphone or camera could be used to listen in.
The only points where your mail can be intercepted are in the senders box or recipients box. And you can combat that by getting a locking mailbox and sending your letters through USPS blue drop boxes. You can also add surveillance of your own.
Even the post office has no way of tracking your letter until it is supposed to arrive at your mailbox. The post office can only track mail pieces that have a tracking number-- and only if the tracking number is scanned as it goes through the network.
TLDR; mail is extremely discreet. Save the post office!