With this simple build bw grant an additional and significant dmg, grant a fast and safe map control and with the Z (+ phase shield) - R combo, will save many times the life of the ally all over the map.
Another important capacity is the safe mercs and boss acquisition using Emerald wind at the right moment.
With polymorph + envenom may greatly increase the potential of any adc.
Gust of healing grant the only one active healing, fundamental during Teamfight

GL HF.