Ukraine joins European power grid, ending its dependence on Russia

> The two countries were previously part of the Integrated Power System that also includes Russia and Belarus. This made Ukraine dependent on Russia's grid operator despite there having been no electricity trade between the two countries for years.

Can someone explain what this actually means and why this is needed? It seems it's not about the actual power supply itself, which the last sentence indicates was already not dependent on Russia. So why does Ukraine's grid need to be connected to that of other countries, whether IPS or CEPS? They indicate it's useful in case the power infrastructure in the country gets destroyed, but I don't understand in what way.