It's a good question. You might be surprised at the answer.
The Bible says that the cross was a demonstration(of God's righteousness). He was revealing the situation as it truly was since long before the crucifixion. God had been forgiving peoples sins and treating them graciously for all time, and had created the world well aware of the suffering that would be caused by his creatures acting disobediently. Since God knows all things, He is intimately aware of the suffering of every man woman and child who ever came into existence. So when Jesus went to the cross, it was to SHOW people the character of God. It was to REVEAL that God was humble and lowly in heart, and that He cared for us and has always cared for us. It was to PROVE that God had not abandoned His people, or failed in His promise.
Total crock, although I realize it is the accepted answer among christians, even though it makes absolutely zero sense. If god wanted to forgive people, he didn't have to invent some bizarre ritual involving 'his son,' another inane concept, since god is supposed to be beyond people and probably doesn't even have any genitals, so the concept of 'his son' could only be metaphorical anyway. Even accepting that, the ritual of having him allow himself to be crucified, a form of execution reserved for the worst of the worst criminals, to take on everyone's sins, still makes no sense. It's god that decided people were sinners and needed forgiveness in the first place. If he decided they should be forgiven, all he had to do was give a little speech about it and--zap--done.
The one thing about it that makes any sense at all is the resurrection. That shows, at least according to religious leaders, that christians don't stay dead. So the sheeple would be happy to A, go willingly to death in support of the church, and B, since defeating death was up to the will of jesus, at least as channeled by religious leaders, keep the followers in line and doing as told. Stray from the path and be doomed---obey without question and be saved. Shit, it's still used exactly in that way, even though it's 2000 years later and we're supposed to know way more about the universe than we did back then.
The reason it had to be a human sacrifice is because that's what the primitive tribes of the day understood. Blood sacrifice goes back to cave days. If you wanted to appease the gods you killed something and gave it to them. Some gods seemed to prefer virgins tossed into volcanoes, other liked theirs tied to an altar and their hearts cut out and eaten. The god of the jews, at least according to what we have written down from the time, dug animals, mostly sheep and goats, as his sacrifices, for whatever reason. The important part was that if you wanted some big juju with your local deity, you killed something or someone for him.
Whoever wanted to be in charge of this particular population figured this out, and so the idea of the human sacrifice that would be bigger and better than any sacrifice before was born. I say the leaders because, while christians still believe the instructions were coming from god, since I don't believe in jesus or god, but am pretty sure that then, as now, there were the sheeple-herders and the sheeple. To be in the know and gain power over the great unwashed, they had to figure out what would impress them most. A super-duper sacrifice that combined both human and deity was the ticket. And that's what they got. A ringside seat to the sacrifice to end all sacrifices.
This is all assuming that the crucifixion ever happened, which it may or may not have. I'm inclined to accept that at least part of the new testament story involves things that really happened, but that it was the leaders posturing to impress the dummies. If true, it seems to have worked. Not only do millions of people believe that the crucifixion and resurrection actually happened, but they have built an entire mythology based on those events with it's own back story, and the myth survives today, when science really should have taught people better. Actually, it really gives credit to the manipulation and control a bunch of dumb desert sheepherders came up with. Kudos, bible patriarchs---excellent story!
That's my take on the crucifixion, for whatever it's worth. ---SherB