I think a physical example communicates more than a spiritual one. Men needed to see the mess sin had caused in their life and how expensive it is to pay for that. Infact i have had experience when witnessing where you talk about the wages of sin is death and spitirual separation, it becomes a bit hard to understand since one has been sinning and not yet died and finds it hard to grasp the meaning of spiritual separation but when you mention something closer physical and practical like burning in hell, it becomes easier to sink in the mind.

I think Jesus saw that no matter how much he preached about the kingdom of God, men still remained difficult and sinful thus needed something more than preaching-a physical sacrifice of his own life.