It makes sense of the transition of weapons as armor got heavier, and gives players a reason to not always take the most damaging weapon. On the face of things, a battle axe (1d8/1d8) is a better weapon to carry than a horseman’s mace (1d6/1d4). But if a target’s wearing plate and carrying a shield, the axe is -3 to hit, while the mace is +1, so there’s a 20% shift in the odds of striking the target. Likewise, the club (1d6/1d3) often looks like a better choice than the horseman’s mace, because it’s cheaper (free compared to 4 gold) and lighter (3 pounds compared to 5 pounds), but it’s an inferior weapon against armors of AC 7 or better.
It can also justify the existence of some of the excessive variety of pole arms - the bardiche is good against unarmored soldiers, the bec-de-corbin against plate, and the guisarme-voulge against ring and chain.
With the loss of the weapon vs armor table, the weapons of choice were bastard sword for a fighter, long sword for a thief, and footman’s mace for a cleric (mage players would argue between the staff for damage and the dagger for flexibility). That blend of weapons would tend to get chewed up by heavily armored opponents, since the bastard sword is 0 against plate if used two-handed and -1 or -2 if used one-handed, the long sword is -1 or -2 against plate, and the mace is only +1. Better would be a bec-de-corbin (+2 against plate), a long sword (thieves are hosed against heavy armor in 1e), and a footman’s flail (+2), while the mage should go dagger (-3, compared to -7 for a staff or -5 for darts).
Note also that the Armor Class Adjustment table was only for actual armor, not for “natural” ACs - i.e. attacking a character in chain mail (AC 5) used the table, but attacking a lizard man (AC 5) did not.