[yabun]

The helium reduces resistance which results in lower vibration of the read/write heads, which in turn allows for reduced space between the platters and the heads, which allows for a reduced space between the platters and more platters per drive. If the same drive was filled with air rather than helium, the increased vibration of the heads would cause them to crash into the platters - which would scrub the disk clean (which would be bad).