Why is this assumption needed in the Universational Introduction?

Question

($\forall$I). If $\gamma$ $\vdash$ [t/v], then $\gamma$ $\vdash$ $\forall$v$\phi$,

If t is

• either an individual constant not occurring in $\gamma$ OR
• an individual variable not free in $\gamma$.

May I know why are these two assumptions needed?