To remind us of the reality of what God did for us. It is all well and good to have happy resurrected Jesus as our Savior and Friend. But we need to be reminded that this is not just a fairy tale or a theological experiment. Jesus really was nailed to a cross and died for us. And if looking at that and contemplating it makes you feel anything it should be grateful.
SD
That Jesus rose again and we can too is just gravy.
SD
This is actually only one (I thiink of three) common depictions that Catholics use. It is common to see Christ depicted in the act of the sacrifice. It is almost as common to see Him depicted as risen (and conquering) the cross (hands and feet not in contact with the cross, eyes open, etc.) and I can't remember the third (I think it's a living Christ enduring the suffering for our sins).
All (or just a plain cross) are acceptable and used largely depending on circumstances (e.g. the parish I grew up in would cover the risen depiction that was on our cross from Good Friday until Easter - It just didn't seem proper to display a risen Christ while contemplating the results of our sin)