Its oxidative properties are supposed to keep the gelatin from taking on bacterial populations. Bacteria are no problem if the gelatin solution is used, and discarded. However, there is a lot of grief with fading of sections, or color change of sections, when stained with the common LM stains. Fading of sections is a complicated problem, but one factor is the propensity of LM stains to react to redox shifts. Therefore, do not use chrome alum in subbing solutions.
Chrome alum makes perfect sense in the path lab perhaps.
That is where it started - with paraffin sections. Those two research specialties have very little in common.