Armor provides a mix of raw defense, resistances, and weight. Lighter armor can have excellent resistances even if it is relatively weaker against brute force attacks. And remember, the strongest attacks are often best evaded, blocked, or parried. Your armor is your last line of defense for when you do get hit.
In general, heavy armor provides superior physical defenses, but the added weight demands a greater investment in stats. So playing a heavily armored tank character can be difficult, if you’re also trying to spread yourself into spellcasting attributes.
In addition to your actual worn armor, there are also magical trinkets that can be found and equipped, providing a wide range of benefits. Some emphasize melee, some magic, others provide powerful defenses or unique offensive bonuses. The full strategy guide for Dark Souls III will feature a thorough breakdown of their specific effects, but keep an eye out for these powerful items as you explore the world, they can be a great boon.