The value of g is maximum at poles and minimum at equator why

it is in the middle

It has to do with centripetal force. The earth spins on an axis. You can think of it like a merry-go-round on a playground. At the center, there's no linear velocity. The farther away you get from the center, the faster you go.

The centripetal force that results from that velocity, combined with the force of gravity, is the net force acting on an object. So at the equator, where centripetal force is a maximum, the acceleration is a maximum. At the poles, where there's no centripetal force, the acceleration is a minimum.