Sending a letter to your future self is like finding a message in a bottle, except it does not come in a bottle, and it`s gonna come through your email but, you get the point.
It can be fun and liberating, you can talk about how you feel right now and laugh about it in the future when all this quarantine thing will be over, hopefully.
You choose the date when you want your message to be delivered so most probably you will forget about it, that`s the fun of it.
P.S. Don`t choose your birthday.