Take a 5th-level barbarian with 18 Str, a greatsword, and a 65% base chance to hit. Reckless Attack improves your hit chance to 87.75%, with a 9.75% chance of a crit. You average 2d6 (base greatsword) +4 (Str) +2 (rage) = 2d6+6 or 13 damage per hit, +7 on a crit, with two attacks. Factoring in accuracy and crits, you average 24.18 damage.What happens if you drop Str to 16 and pick up GWM instead? Base chance to hit drops to 35%, but Reckless Attack brings it back up to 57.75%. Average damage per hit is 22, +7 on a crit. Crit chance is still 9.75%, but in addition to damage, you get a bonus attack on a crit; with two attacks, this has an 18.55% chance to trigger each round. (Bonus attack also happens on a kill, but I'll assume that cancels out the times you don't have a bonus action free.) Work it all out and you get 29.26 damage per round, or a 21% increase. That's the baseline for what you, the barbarian player, can get from the PHB on your own, and it's already quite respectable.