The highest note that is playable on the keyboard is also known as high C. People know this as C8. There are some notes that singers were able to sing well that are considered really high. Some have associated the ability to sing high notes as being a good singer.
For example, there are a lot of people who consider Mariah Carey as one of the best singers because she was able to sing a G7 before when she sang the Star Spangled Banner back in the year 2003. High notes can be fun especially when they are matched with other notes that will all complement each other.