The mercy seat is an object that rested on top of the ark of convenant and was really associated with the day of atonement in the Old Testament, the ark and it's mercy seat represented the presence of the Holy Spirit and the mercy seat was made of pure gold. These are most reason why it's was very important because its like the presence of the Holy Spirit.
The mercy seat is also mentioned in the New Testament. Hebrews 9 discusses the Holy Place that included the Ark and the mercy seat.

Thanks for sharing, God bless you Amen.