What do you believe is the most important field in which the use of UAVs has become indispensable?

It is in the field of defense and military (reconnaisance).

There are many important fields in which drones have become indispensible. For me a major filed would be in the medical field, where drones are used to delvier inportant medical supplies and blood samples. This has become a necessicty for deilvering medical supplies to rural areas in Africa for example. Secondly, drones have become useful in mapping urban development and landscape changes. Through digital elevation models, researchers are able to map hydrology for example, which can prove usefull in solving water management problems in drainage basins. 

I think that would be Search and Rescue