The gpio pins have low tolerance. Apply more than the 8mA or 3.3v and you permanently damage your board. LED's usually require 20mA to achieve a good brightness

The gpio pins have low tolerance. Apply more than the 8mA or 3.3v and you permanently damage your board. LED's usually require 20mA to achieve a good brightness so you have to create a safe circuit using a bipolar transistor and the 3.3v power pin which supplies way more current than a gpio pin. If you hook up that transistor wrong or short the circuit it's game over. I actually think you can even damage the board if you create a short between the gpio pins and ground. So any kind of circuit requires a resistor, carefully reading the system manual and data sheets for your components and using ohm's law prior to hooking up components.