Why should the cubic equation in volume, obtained from Van Der Waals equation of state, have equal roots at the critical point?

The video [Edmerls —
Derive Equation for Critical Constants in terms of Vander Waal's Constants. | Physical Chemistry](https://youtu.be/qyBK4Gio80w) (YouTube) provides a derivation of the relationship between the van der Waals constants and the critical pressure, temperature, and volume.


It involves rearranging the van der Waals equation to form a cubic equation in the volume $V$ and comparing it to $(V - V\_\mathrm{crit})^3 = 0$, where $V\_\mathrm{crit}$ is the critical volume.


Why does the equation need to have equal roots? And can it also have equal roots for other fixed values of pressure and temperature (apart from the critical pressure and temperature)?


