What is the main function of Fire hydrant &the distance required between the fire hydrant and the buildings?

12 m from the building

75 m between two points

That requires an iterative process in design calculations involving zoning that we can just say in a matter of mental discretion I suggest you read NFPA1 Fire Code and the fire code within your local municipality for guidance because that varies depending on zoning requirements provided by your local city engineering office.

It shall be the design of the structure of the building, external walls of fire-resistant materials. To achieve the principles of protection of the outer spread of the fire building must be a safe distance from other buildings site to reduce transmission of fire, if a fire broke out - God forbid - of Alambny- or him.

And increasing the distance the greater the number of openings Kalnuaz, porches, or I resist the outside walls of the fire, according to technical charts, engineering approved by the competent authorities Kalpldeh, Authority for Standardization and Metrolog

fire hydrant is external perimeter fire fighting , the distance according to LPC is6 meters at least and to NFPA it is meters 

Distance to the building and to the other hydrant :

For Detached One- and Two-Family Dwellings.

1. The maximum distance to a fire hydrant from the closest point on the building shall not exceed 600 ft (183 m)
2. The maximum distance between fire hydrants shall not exceed 800 ft (244 m). 

Buildings Other than Detached One- and Two-Family Dwellings.

1. The maximum distance to a fire hydrant from the closest point on the building shall not exceed 400 ft (122 m).
2. The maximum distance between fire hydrants shall not exceed 500 ft (152 m).