Men are sinful in nature, ever since Adam and Eve left Eden. Spilled blood was needed to atone for our sins. Jesus is the Lamb of God. He lived a sinless life. He was the perfect sacrifice. He was the last blood sacrifice. Accepting His sacrifice, that he died for your sins, grants you salvation.

Sidenote: This is also why Christians should be indifferent to Jews looking for red heifers to sacrifice. Jesus fulfilled all the old laws and prophecies with his death and resurrection. Animal sacrifices are idolatry at this point. It's a strange case of doublethink, were most Christians falsely believe that modern Jewry is the succession to ancient Israel and follows its Old Covenant (even though it doesn't), but looking for red heifer is some sort of sign that Jewry is returning to its roots (which is actually the Talmud and its derivative works, not the Old Testament) and will eventually turn to Christ.