When examining the importance of the crucifixion we need to consider two separate aspects; the significance of the Lord's death and then the significance of the mode of death chosen. This can be justified from Php 2:8 where the latter is seen as an extension of the former. By taking this approach we shall see that the crucifixion has a theological (or God facing) significance as well as a evangelical (or man facing) aspect. I will then suggest that the two paths of thought combine in the blood of Christ.
The easiest way to establish the need for the crucifixion is simply to show that it was prophesied extensively in the Old Testament and that therefore the Messiah complied to fulfill scripture. For example Psalm 22 is one of the most graphic first-hand accounts of crucifixion ever written; some nine hundred years before the Lord was crucified and two hundred years before crucifixion was invented. Isaiah also detailed aspects of the Lord's mode of death and Daniel made clear that the Messiah would die.
However we are told that the Lord's manner of death had been planned by God. Indeed we have reason to believe it had been planned from before the start of creation. Therefore we have to view the Old Testament prophecy not as a prediction but as the declaration of a decision. So, to understand the crucifixion we cannot simply say 'it had to be that way' but instead we must attempt to examine why, given all the alternatives, it was chosen to be that way.
The need for the Lord to die is established on the basis of the justice of God. It is axiomatic that sin is to be punished by death. Further we are told that everyone has sinned. In the Old Testament economy a sin sacrifice of an animal was allowed to substitute for a death of a person thus establishing the notion of substitution. However an animal for a person is hardly a 'just' trade and we are told that the Lord had to die not simply to make the sacrifice permanent but also to render the forgiveness offered in the Old Testament just. Thus, from a theological perspective, Christ had to die so that God could be just in not executing us.
I suggest that the need for the manner of death is actually given to us by the Lord Himself: "And I, if I be lifted up from the earth, will draw all men unto me." Whilst this reference may seem a little opaque we are specifically told it refers to the manner of death. Crucifixion was one of the most graphic, grisly, public and extended modes of death available. This gave the people at the time and us today a particularly visual and easy to understand picture of what we put the Lord through. By seeing Him there upon the cross the decision we each have to make is simple - the details are brutal but readily comprehensible.
There is however an important link between the theological and evangelical; that is the blood. We are told theologically that the mode of death had to include blood loss and that the life of a person was even deemed to equate to his blood. We are also told that an acceptance of the blood of Christ is a basis of salvation and there is not doubt that the blood is a particular graphic and emotive part of the crucifixion. Indeed we are even told that in a time to come the piercing done to the Lord will be a basis of recognition for repentant Jews.
In conclusion; a seemingly mindless act of barbarity enacted in a remote outpost of a huge empire was actually the fulcrum upon which the justice of God and future of mankind was finally balanced. We will probably never fully understand the cost or value of this sacrifice until eternity. Until then we have been left some important clues which will hopefully inspire us to appreciate Jesus more fully; and to share that with others.