If you truly have a first choice yes. If not then no

Is it worth sending one if your child only applied to 2 schools?

We sent the letter once we completed all of the visits and testing. My child applied to multiple upper elementary schools and we (parents) sent the letter once we were sure of our first choice.

When in the process is such a letter submitted?

For little kids below 4th grade parents. After 4th I would have parents and the child write separate letters.

Is this done only for pre-school/K or at all levels? If it's done at high school, who writes it, the student or the parents?