The Z80 is an 8bit CPU designed by Zilog.
All NeoGeo systems use it as a coprocessor to handle "tracker music" playback. It only has access to the YM2610, M1 ROM or .Z80 files, and its RAM.
Since it doesn't share any direct memory zones with the 68k, it can't be used to perform or speed up gameplay-related tasks.
See Z80 Memory Map.