Why does an Instrumentation Amplifier need a reference?
As you can see, if you look at the schematic for the INA826 instrumentation amplifier, the reference input,  unlike the -IN and +IN inputs, is not high impedance: 



So, adding a buffer makes it more generally useful for evaluation. If you have a low impedance source for your reference (for example, ground) you don't necessarily need a buffer. 

The EVM provides the PCB patterns to add such a buffer (U2 on the schematic), but it is supplied unpopulated.