MTV hydraulic brake actuator

We've recently within the past few years gotten FMTV's. I have done tons of work on them but there is one thing that I do not understand. Why do they have brake reservoirs and an air over hydraulic booster tucked between the step and the front bumper on each side? My new Wrecker even has it and it's equipped with an LTAS cab which is tilted by an electro-hydraulic pump. I could probably figure it out on my own if I was around them longer than a weekend a month, 2 weeks out of the year but we're too busy doing other stuff on those days. Please help because I'm confused.

The front axle brakes are hydraulic actuated, why I don't know. Instead of the usual air chamber forcing the wedges in to actuate the shoes, these have hydraulic cylinders to force the wedge. The Caiman chassis that were scrapped here were 50/50 on air and hydraulic.

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The upper chassis has the hyd cylinder, the lower chassis has the traditional chamber in the same spot.

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I think this change to hydraulic front brakes happened around 2005.