EagleCAD Device Dimension layer
When I went to build the board I was surprised to see that instead of just having the pads in the right place for a shield it also had a "dimension" layer and drill holes.


Its uncommon to include dimension in the part definition but practical in some cases. When the gerber output for PCB production is generated, it doesn't matter where the dimension information comes from. 


  I am now trying to understand what that means for me when it comes to the actual board dimension since the component already has what I could consider board dimensions.


You don't need to (and shouldn't) draw another dimension outline in this case. As I already pointed out, when looking at the raw gerber data it won't make any difference if it was placed as part of the component or separately.