Coal is formed when layers of plant matter accumulate at the bottom of a body of water and are protected from biodegradation and oxidization. Over time, the properties of the original material change. What evolutionary significance, if any, does this have?

The evolutionary significance is that coal does not change the properties of the plant from that time even though the plant might have evolved from that time. That's why you can study coal to find out how the type of plant was at that time period and how it has evolved until now.