Poor posture has can have many effects depending on the area which is bad. Posture is not only bending forward, but also side to side such as 1 shoulder higher than the other or a hip being higher.
Now lets say we go with right hip being higher up than left. This could lead to tighter muscles on one side causing them to remain in tension all day, and lower development on the other side. This then could effect the knees, ankle, walking methods. This then causes more pain or issues, then leads to worse posture, etc, etc and on we go.