Why optocouplers for MIDI Out/Thru?
Optocouplers are used for galvanic isolation. In most cases, this protects against dangerous voltage differences, but in MIDI inputs, it just prevents ground loops.

However, the optocouplers used for the MIDI outputs in the linked schematic do not provide any isolation whatsoever, because the grounds and +5V power supplies are connected together (they are the same). Those optocoupers could be replaced with a simple transistor, or (because no amplification is needed) with a piece of wire.

That schematic is not the work of a competent designer. Ignore it.