The magazines consist of a 10-issue run that each give an additional rank of the Barbarian perk, which raises the critical damage of unarmed or melee attacks by 5%. This stacks together to gain up to a 50% increase in melee or unarmed critical damage, if all comics have been found.
Issue #09 "In the Lair of the Virgin Eater" is the same comic book that is on the cover of all Grognak the Barbarian comics in Fallout 3 and Fallout: New Vegas.
There are actually 11 Grognak the Barbarian comics to find across the Commonwealth, as Issue #05 "Demon Slaves, Demon Sands" has 2 copies, one in Vault 75 and one rewarded for the quest Short Stories. Regardless if one gathers all 11 or misses an issue, they will get the Barbarian perk ranked ten levels. However, getting 2 copies of "Demon Slaves, Demon Sands" will not rank the player character ten levels if they are retrieved before getting all the others. All separate issues are needed to rank 10 in the Barbarian perk.