In short, anyone enjoying or overseeing a vista or view. They are generally used for long-range terrestrial observations or astronomy over a long period of time when the use of both eyes is more comfortable than using a telescope. The use of large objectives results in a large binocular, which necessitates the use of a good tripod for reasons of weight and to hold them steady under high magnifications. Using large objective results in great light grasp, the practicality of high magnifications and better resolving power and so long-range study of detail is improved. There is more.
Larger objectives allow the use, with minimal errors, of long focal lengths.
Everything in optics is a compromise somewhere along the line and relies much upon the designer's art. Needless to say, such binoculars need very sturdy tripods to obtain the best results. The origin of big observation binoculars was with the military which used them for the early spotting of enemy ships and for artillery spotting but much of this duty is now carried out with the use of satellites and electronics. Because of the outdoor environment in which these instruments are often used, they are often filled with nitrogen to prevent fogging from moisture and to prevent long-term oxidation of the optics. Long focal lengths to minimise colour dispersion and improve depth of field.