Why did Jesus need to be resurrected?
"The price is paid" the old song says, of the events of the cross and Jesus' death; our wrongdoing has been dealt with and the ransom for our freedom paid. Why, then bother with something as costly as the resurrection? What's the point?
"I take no pleasure in the death of the wicked, but rather that they turn from their ways and live."
So, bearing in mind that Jesus never sinned and therefore couldn't have died for His own wrongdoing, how much more would God desire Him to be alive? Add to this statements like, "This is my beloved son..." (at Jesus' baptism, see Matthew 3:17, Mark 1:11 or Luke 3:22 and at the transfiguration, Matthew 17:5) and you get an idea of God's preference for life and not death.
Had Jesus remained simply a martyr, He'd be no different to any other major political leader or religious icon.
Josh McDowell, "Evidence that demands a verdict"
"All but four of the major world religions are based on mere philosophical propositions. Of the four that are based on personalities rather than a philosophical system, only Christianity claims an empty tomb for it's founder. Abraham, the founder of Judaism, died about 1900 BC but no resurrection was ever claimed for him.
The original accounts of Buddha never ascribe to him any such thing as resurrection; in fact, in the earliest accounts of his death, we read that when Buddha died it was "with that utter passing away in which nothing whatever remains behind."
There is no trace in the Pali scriptures or commentaries (or so far as I know in any Pali book) of Sakya Muni having existed after his death or appearing to his disciples.
Mohammed died, June 8th 632AD at the age of 61, in the city of Medina where his tomb is visited annually by thousands of devout Muslims. All the millions of Jews, Muslims and Buddhists agree that their founders have never come up out of the dust of the earth in resurrection."
So, the resurrection marks Jesus out as unique among all of the major world religions. The resurrection validates Jesus' own claims to be God and have the power over death itself - God gave Elijah and Elisha the power to raise people from the dead, Jesus raised people from the dead and He gave his disciples the power to work the same miracle. It's only consistent that He, Himself, should also be raised.
His death wasn't deserved. He never sinned and yet He died a sinners death on the cross. He had no right to remain dead and (at that point) justice had been done, hence the need for a resurrection. More than that, He also predicted it. It validates what He said about other things; Jesus proved His word to be trustworthy in this regard, so, it validates His other claims and statements.
It proves who He said He was - the Son of God. Like, "I am the one and only Son and death cannot hold me - watch - three days in the grave and I shall be raised." It proves the power of God - if death is no barrier or obstacle, nothing we face is... "If God is for us, who can be against us?" (Romans)
The Jews of the time were expecting a Messiah who would overthrow their current political regime. Jesus is the Messiah, but, they were looking for the wrong thing. His death and resurrection demonstrate (and fulfil) messianic prophesies from the old testament days, proving His claim upon the title. In a sense, He had to remove their wrong ideas and replace them with the facts. His death shattered all hopes of the political victory they were looking for, the resurrection replacing them with even greater and brighter things - knowledge of what the Messiah really is about. It reforms the broken pieces into a correct picture once the erroneous pieces had been removed.
When pressed for "a sign" Jesus said everyone would have only the "Sign of Jonah" - the old testament prophet who spent three days inside a fish before being spat out. An interesting point once suggested says that he actually died and three days later was raised to life, though (IMO) the symbolism of the account is enough to foreshadow Christ's death and resurrection.
Jesus spoke of a future resurrection coming to us all. His own is a foretaste of what is to come - a demonstration, or test drive, to show what will be.
The resurrection changes things from simply trying to do one's best to follow the sayings of a dead leader [however great they were] to something where the living leader walks alongside to show the way, to aid and support the believer in all that lies ahead.