1. You can put very knobby and wide tire which means more grip
2. You have more tire options than on 19"
3. Better for trail riding - 18" tire is fatter so you can also go with less pressure and is more resistant for pinching
4. 18" tire is more flexible and you can have more traction on a hard surface like rocks etc. with lower pressure
5. Main wheel weight is closer to the rotation center which means less power to rotate the wheel -> longer range than on 19" with similar tire. 
Features
- Milk Racing black matte rim 18”x1.85
 - OEM SurRon hub
 - SM silver 2.6mm/12G spokes
 - lacing with 3-cross spoke pattern
 
- Milk Racing 18" wheel (rim, hub, spokes, nipples)
 - inner tube standard 3.50
 - rim band
 - rim lock
 - all bolts and hardware (stainless steel)
 - spoke wrench for spokes tightening
 - Torx key for disc brake bolts (disc brake need to be swapped from OEM wheel)