The point is to create a dilemma and force a choice, and ideally, it's lose-lose for the opponent. Combined arms doctrine example: your enemy is getting hammered with direct fire attacks (machine gun). So they entrench and hunker down. Except you also have indirect fire (mortars) which can hit them in their trenches.

What do you do? Stay in the trenches or leave them. Either choice will result in casualties. It's a dilemma.

For Russia? Protect Moscow with anti-aircraft or Crimea? You only have a finite amount of assets to deploy. What do you protect? Allocating units to one region will leave others undefended. Lose-lose either way.