Why do you think scientists need definitions for the different types of planets?

Because each planets are different and has different things they do

Scientists need definitions for different types of planets to put them in specific categories. Some planets have similar qualities, but every single planet is unique in their own way. You can't just say, this is a gas planet, because there will always be something else that differs from other gas planets. It's just to help organize them into specific little things that are in the grand skeme of the universe, but are still their own little world that may or may not inhabit other species.