Why is there a mercy seat on the ark?
The mercy seat is on the ark because the law inside it is so demanding that only God's mercy can satisfy its requirements. The cherubim's wings met over the box as a covering of God's mercy, which alone answers to the law's demands. So the mercy seat exists to meet the law's demand through divine mercy.
Les explains that the cherubim were two angelic-like beings whose wings literally met over the middle of the little box made of pure beaten gold. Those cherubim indicated a covering of God's mercy, because God's mercy is the only thing that could answer to what was inside the box—the law. The law is so demanding that the only way anyone can even come close to satisfying its demand is in God's mercy. That is why the mercy seat was placed on the ark. Les also connects this to Romans 3:23-24, where Paul writes that all have sinned and come short of the glory of God, and are justified freely by His grace through the redemption that is in Christ Jesus. He notes that the more interested a person is in real Bible study, the more glorious the tabernacle becomes.