kinematics - What is the difference between these two ways to calculate average velocity?

Average velocity:

$$v_{\rm avg,1}=\frac{v_{\rm final}+v_{\rm initial}}{2}$$

and average velocity:

$$v_{\rm avg,2} =\frac{\rm total\;displacement}{\rm time \;taken}=\frac{\Delta x}{\Delta t} $$

What is the difference between them and when do we use them?