You're right, that does sound like a really stupid question (jk)
Mostly to prevent things spilling on the components, or accidentally touching the components (a must if you have pets/kids), to provide good cooling (airflow), to prevent dust buildup, to keep the stuff in an organised small space (like cables).
While alienware cases have a look that many people like they generally have poor airflow which is what you need when pushing your hardware to its limits.
I prefer a more simple clean look with good airflow over a case that looks good.
If you can get both, that would be ideal but looks are a personal preference.