There are lots of reasons for the MIDI IN.. Here are a few examples..
Chaining.. The elemental chain is a single-link chain. The MIDI OUT port of one device is connected to the
MIDI IN port of a second. In this configuration, a key pressed on the first unit will cause both units to sound. Pressing a key on the second unit, however, only causes the second unit to sound. Many instruments may be chained together using a series of single links to connect the units. In this case, the OUT of the first unit is connected to the second, the THRU of the second is connected to the
IN of a third, and so on. If all the units are set to receive on the same channel, pressing a key on the first one will cause all the units to sound. Pressing a key on any of the other units will only activate the sound of that unit.
ALSO A MIDI LOOP... A MIDI loop is a special configuration of a MIDI chain. The single element loop is made of two interconnecting links. This was the configuration used in the debut of the MIDI system. The OUT port of the first unit is connected to the
IN port of the second, and the OUT port of the second is connected to the
IN port of the first. In this case, as described earlier, a key pressed on either unit causes both units to sound, provided they are on the same channel. A MIDI feedback loop does NOT exist here, as the data going into the second unit from the first is not duplicated in the OUT port of the second going back into the first. Here, we have two one-way links connected, not a multi-link chain.
The nice think about your controller is the fact is allows for you to Simply Adjust all your presets. Check out this video..
http://en.440tv.com/video.php/v/2366/Axiom-61-Keyboard-MIDI-Controller-by-M-Audio
Also in the video he talks about a GOOD YAHOO Group that is another Great Resources.