It’s a good question and an important one. @Jesse_Means_God_Exists is correct that it is established in the sacrificial system for the atonement for sin. In Exodus 29 God gives Moses very specific instructions for atonement with a system of sin offerings—sacrifices involving the blood of firstborn animals.
It shows the gravity of sin that blood is required for atonement. God’s instructions include directions for the priest(s) and their holiness so that they may represent the Israelites before God in presenting the sin offerings. In our case Jesus is both the priest who is holy before God AND the spilled blood of a firstborn without blemish (perfect).
We believe God to be both just and consistent. To fulfill the requirements of the law that would make us right (through Jesus) before God He who was without sin paid the penalty for my sin with His blood.
To more fully understand this spend some time in Exodus, Deuteronomy, Leviticus to see how explicitly how the Israelites were to live out the mosaic covenant (the law). Then study Romans to understand the limitations of us concerning living out the law and how Jesus death and resurrection is/was required to satisfy the law and establish the new covenant.
Another aspect is that covenants were also sealed (not sure if that’s the right word) via the blood sacrifice of an animal. Once again as a symbol of the gravity of a covenant. A promise so permanent that a life was sacrificed to secure it.
All of the OT leads to the cross. When God spared Isaac from Abraham’s hand and provided the ram, that ram was Jesus being sacrificed for all mankind. Once I read through the OT with that in mind I saw the work of Jesus everywhere, as well as a human history of sin and depravity that time and again proved our need for a savior.
Hoping I’m making sense with this. Truly, in Romans Paul makes a brilliant case for all of this. It would be a very beneficial study for understanding this.