Why directional drilling is required?

Hit targets that can not be reached by vertical drilling. Drain a broad area from a single drilling pad.  Increase the length of the "pay zone" within the target rock unit.  Improve the productivity of wells in a fractured reservoir.  Seal or relieve pressure in an "out-of-control" well. 

Install underground utilities where excavation is not possible.   

There are a few reasons why. You can avoid obstacles such as an impermeable strata or environmentally protected lands. Another reason can be the rights to the resource. You may not have the rights to extract the resource directly above so directional drilling from an adjacent property is required.