The aim of this work is to study the situation of a $\mathcal {C}^\infty $-manifold endowed with a projective structure $\mathcal {P}$ and with a system of symmetries leaving $\mathcal {P}$ invariant. Under the additional hypothesis of projective homogeneity, the author ifies all such spaces and exhibits a geometrical interpretation for inessentiality of the symmetric projective structure using techniques of affine differential geometry.