From the Game Manual:
DMI: The overall player index, DMI, gives a very rough indication of how good your player is. This is a good way to check that your player is getting training, but is otherwise a lot less meaningful than it first appears. Be warned!
This can be an indicator to how low, medium, or high a person's level of Game Shape is. For example, a player with 8 GS and 162k DMI might be in slightly better from than an equally good player at 9 GS and 160k DMI.
For me, the importance is to know that my trainees are getting training, and to also be able to compare my players with the players of my opposition, and how their DMIs match up.