obedience and blessing
thats another test of obedience. If God needed the blood to know the residence of the Iseralite who the identified the first born of the other houses without blood on thier door post. God wanted to know those who would heed to his commands and instructions. Even some the Egyptain that obeyed the commands were saved. Another case of such tests is recorded in [number 21:4-9] when the Iseralites angered God and he sent venomous snakes to them. When they understood thier flaws and asked for forgiveness God did not just send the snakes away neither did he remove their poisons, which nothing would stop him from doing, but he wanted to test their obedince, look and live, if anyone bitten by the snake looks up to moses's bronze snake he/she would live. One thing i am preety sure is that some people died from those snake bites because the didnt want to be submissive to the government of God headed by moses. Gods way differ from that of men, so when give commands if may not go down well with us but blessing come when we obey those command. Hence comes the adage "their is no testimony without a test.