Question

why the energy of an object is increased when raised through a height

Answer 1

to lift a mass m some vertical height h in Earth's (any) gravitational field, the g force must be overcome — by some force doing work on the mass, against g — and this is what makes the potential energy PE of the mass to increase. The mass may already have some PE, depending on its elevation in the gravitational field.

Answer 2

The potential energy of body increases with height.
As potential energy=mgh
As m (mass) and g(gravity) are constant we can say that potential energy is directly proportional to height
Hence with increase in height it also increases.