Going purely on the circuit as shown, none at all.

However, there are reasons why it may be appropriate; eg,

Although only low-current LEDs are shown, Eric might be planning on using higher currents once he gets it working...;

If Eric is making a general experiment board, buffers are good so that any little "accidents" don't damage the main processor;

Many processors have a maximum current rating per pin, and another total rating for the whole chip - which means that, although each pin can individually drive a LED, you can't have all pins simultaneously driving LEDs;

That's 3 that spring to mind.

Perhaps Eric could explain why he used them?