Mounting the compressor (s) up front was common on larger power (lots of examples running today such as UP 844, GTW 6325, PM 1225, NKP 765, etc.) but mounting it on the smokebox front appears to originate with pre-Super Power locomotives running in the mountains (like C&O's 2-6-6-2s).
The main benefit I can see is that it allows for a larger reservoir on the fireman's side than is possible otherwise (those C&O 2-6-6-2s were bring long strings of loaded hoppers down steep grades, and of course D&RGW's own standard-gauge examples faced some steep downhills). So I'll speculate that someone thought the K28 would benefit from bigger lungs.
JAC