What is the importance of point of zero shear or S F sign changing point?

Zero SF occurs at location of maximum moment in a member.

structurally the zero shear corresponds to an extreme value at the moment function, hence the shear function is derived from the moment function, therefore we cannot make stop joint at that location.

My point of view the zero shear location will give only the location of minimum shear reinforcement.

It's important to determine Pour Strip or casting stop joint