Answers "if things go badly, how badly?" - the question VaR refuses to answer.
It is coherent, in particular subadditive: the expected shortfall of a combined portfolio never exceeds the sum of the parts, so it respects diversification. That is why regulators have moved toward it.
The cost. It depends on the shape of the tail, which is the hardest part of a distribution to estimate from limited data. It is the better measure and the harder one to compute reliably - a trade-off worth naming rather than glossing over.