This allows things like mega evolutions and forme changes to work without their normal item restrictions. 
Some examples include: 

- Zacian-Crowned and Zamazenta-Crowned can now run other items (ex: Life Orb, Z Moves, Choice Band)
- Mega Evolutions can run other items as well, can be used from the getgo, and you can use as many megas as you want (ex: Mega Rayquaza with Heavy-Duty Boots + Mega Gengar with Life Orb on the same team)
- Cramorant-Gorging can be used immediately, which means once per switchin, when it takes an attack the attacker loses 25% HP and becomes paralyzed
- Primal forms can run items as well