Why does a Passover lamb require blood?

A Passover lamb required blood because the shed blood had to be applied to the doorposts as the sign God would see, causing Him to pass over the house and not let the death angel strike the firstborn. By faith, that blood kept them secure.

Les explains that the Jewish families stood in their hovels around the kitchen table with the Passover lamb roasted, partaking of it for strength for their journey. They did not shake with fear all night because by faith they knew the blood on the doorpost kept them secure; the death angel would never strike them. The whole idea was the shedding of the lamb's blood so it could be struck on the doorpost. Paul says in 1 Corinthians 9 that Christ was our Passover, and if there is a Passover lamb, there has to be blood. Romans 3 is the doctrinal ground, not just an example.

1 Corinthians 5:7

Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:

Why does there have to be blood if Christ is our Passover lamb?

The Passover required the shedding of the lamb's blood, which was applied to the doorposts so God would see it and pass over, and the death angel would not strike the firstborn. Therefore, if Christ is our Passover lamb, blood is necessary because the whole idea was the shedding and application of the lamb's blood.