Why can't I use crewMember.name?

This code was used in the example in this exercise: for (let crewMember in spaceship.crew) {
console.log(${crewMember}: ${spaceship.crew[crewMember].name})
My question is, why do you use a bracket around crewMember in the last line? It is not a two-word string, nor does it have special characters or a space.??