Do you remember what it was like to get a letter in the mail as a kid?
You should, because it was awesome. You were definitely hoping there was money in there, but even if there wasn't, it was really cool to have something in the mailbox written just for you.
That feeling doesn't go away just because you're an adult. People love getting handwritten notes, because getting a handwritten note means that someone's given you something even more valuable than that 20$ check from grandma: their time.
And you can't take time back. Or make more of it. Warren Buffett makes more money than almost anyone on earth, but he can't make time. So when you give someone else your time by writing them a note, you're giving them a lot.
Writing notes accomplishes two things:
1. It keeps you in that person's mind. You could leave the greatest first impression in the world, or deliver an incredible sales pitch, but you could still get lost in the day-to-day of that person's life. Writing them a note cuts through that and more importantly, lets them know that you value them enough to spend time on them.
2. If you put something in the note, even if it costs you next to nothing, it'll help them remember you even more. I write 3-5 notes to people every week, and when I was in Tennessee, I'd put a lottery ticket in every one. I had a lot of people come back to me weeks after the fact and say "hey, I got your lottery ticket. I didn't win nothin', but it was fun!".
Write notes for clients, friends, family members, or anyone else you want to know how important they are. I promise: they won't forget it or you.