Have a look at this:
Most GPIO pins of SoC's can drive much more current low than high, that is when the GPIO drives a pin low it does it with more "force", than when driving it high.
So its better to connect a resistor to the power supply, then drive it low with a LED in series, than to tie the resistor to GND, then try to drive it high with a LED in series.
If you drive the led in the first way, you can send several milliamperes through it, if you do it the second way, only a fraction of a milliampere.
In Crostons example there will only flow (when using a typical red LED, which has a forward voltage of about 2 Volt) about 0.3mA through the LED, only the most efficient LED's will work, and then still be quite faint. With the method shown in the above link (470E tied to 3V3) and also a red LED there will flow about ten times that, or 3mA, which is a safe value, and will make a LED of normal efficiency shine okay, and a high efficiency LED shine bright.