Not needed.
I retired a Macbook Pro last year after 12+ years of travel and transport. It always rode in a soft-side padded zipper sleeve inside my backpack. To this day the computer remains unmarked. It looks new.
There is a huge variation in hard-shell case design quality. Some designs ignore the critical air intakes where the chassis and bottom plate meet, like on my Macbook Pro 14":

Overheating due to a blocked or restricted air intake can render your computer beyond practical repair.
Another reason against: particles of dust or dirt can be caught between the shell and the computer. Eventually that, combined with the vibration of normal use and transport, makes scratches that the shell was supposed to prevent.
I try not to pay money for "protection" that can cause damage. One man's opinion.