No quad channel on z370. You CAN use all 4 sticks but in dual channel only. The ram will default to the slower speeds. You can manually overclock the first set to match the second. You may have to bump the voltage up for stability.
They allow your cpu to handle more parallel workloads. Most times threads=cores unless hyperthreaded then it 2x the number of cores. More threads allow for faster editing of videos, encoding music, unzipping compressed files, medical research, etc.