5. Why might it be useful to determine the elements that a planet or moon is made up of?

because we will know more about out solar system

One of the main interests in science relating to space, is finding other planets that are similar to Earth (similar atmosphere and can sustain water and in turn the potential of life)

So if we are able to determine composition of the planet, we might be able to see if it can or does have water on it - which could mean life can survive on the planet. Which opens up options of populating another planet, or discovering a new life-form / living organism.