Complex sentence: This was a significant difference in destructive power, since 8-inch shells were over twice the weight of 6-inch shells, but light cruisers were nevertheless useful for fire support and as fleet escorts, and heavily used.
Split into simpler sentences: This was a significant difference in destructive power, since 8 "shells were over twice the weight of 6" shells. Light cruisers were nevertheless useful for fire-support and as fleet escorts, and heavily used.