Comets come either from the Kuiper Belt, an area full of space objects around the orbit of Pluto, the dwarf planet, and they also come from the Oort Cloud, an area 100 times farther away. Comets are believed to be life messengers because they are rich in water, Carbon, Nitrogen.
Due to the Carbon compound in them their core is pitch black. They are messengers from another era because some of these space travelers were formed about the same time as Earth. Sutdying them would reveal new info about the conditions of those times. Still, was a bombardment with comets enough to get life starting on Earth?