What is the purpose of Strength, Intelligence and Dexterity in Path of Exile?
Each of the stats has some form of defensive scaling but only strength directly increases your damage and dexterity will increase your accuracy (thus amplifying your damage indirectly):
Every 10 Strength grants
- An additional 5 maximum life
- 2% increased melee physical damage
Every 10 Dexterity grants
- An additional 20 accuracy rating
- 2% increased evasion rating
Every 10 Intelligence grants
- An additional 5 maximum mana
- 2% increased maximum energy shield
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As you've mentioned in your question you're already aware of the fact that all the stats are required to equip certain pieces of armor and/or skill-gems.
Here is where PoE is somewhat different from most other games (which can be confusing):
Very often it's a good idea to build opposite stats on character builds (e.g strength on mages, intelligence on warriors etc.).
If you look at a witch as a starting class for example you wil usually have no trouble obtaining the intelligence required to equip a certain piece of mage armor or a spell-gem. However maybe you want to use a fire spell (those usually have some strength requirement) and often times you'd have to actively invest into strength to equip and upgrade the gem past a certain level.
For aboout 90% of builds you want to have just enough primary stats to equip all your required skills and armor at their optimal ranks. There are some builds that are focussed on maximizing STR/DEX/INT but they're definitely not the majority.
Every 10 Strength grants
- An additional 5 maximum life
- 2% increased melee physical damage
Every 10 Dexterity grants
- An additional 20 accuracy rating
- 2% increased evasion rating
Every 10 Intelligence grants
- An additional 5 maximum mana
- 2% increased maximum energy shield
Remember that the skill tree offers very generous ways to invest in stats that don't usually fit within the class archetype you are gunning for. There are a fair amount of notable passive skills that are easy to path to that grant stats, including notables providing off-class stats not associated with that class. For example, the image below has notables offering +30 int and +30 str, conveniently next to the Duelist and Ranger classes, both of which are primarily dex-based.
Gearing items like weapons and armor can have prefixes that add attributes, and jewelry like amulets and rings can provide implicit attribute bonuses. Certain Unique items may also provide additional benefits to your attributes, such as the Shaper Unique Shaper's Touch.
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