Very simple. It's purpose is to raise the engine's rpm's without having to be in the cab with your foot on the gas pedal.
Why?
To run PTO driven equipment for one.
To run engine belt driven hydraulic equipment for another
On 4x4's they were common if you used it to plow snow with. The hydraulic driven plow angle and lift is more efficient the faster the engine revs.
If you want to sell the one on your 71 because I was kind enough to explain this I'll buy it. I need one for my 72 GMC plow truck. 
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Currently on or near the homestead:
67 Chevy SWB 2WD stepside 350/3 on tree (Pat's)
67 GMC SWB 2WD Fleet 402/auto (Brian's under construction)
67 Chevy 3/4 ton 2WD 402/auto (Business Hauler)
67 Chevy 1 ton dually 2WD 396/4 speed (Former business hauler, Needs TLC)
68 Chevy 1/2 ton Suburban 2WD 250 six/3 on tree (Brian's Needs TLC)
70 Chevy 3/4 ton 4WD 350/4 speed (Pat's - Disguised as a 68 GMC)
71 Chevy SWB stepside (Crushed by tree - parts donor)
72 Chevy 3/4 ton 4WD (Parts donor)
72 Chevy 3/4 ton 4WD Suburban (Parts Donor)
72 GMC 3/4 ton 4WD 292 six/4 speed (Mine - Disguised as a 67 GMC)
81 GMC 4WD Dually Dump Body 350/4 speed (Business Hauler)
82 Camaro Z/28 355/Super T-10 (Pat's toy)
93 Caprice 9C1 (Brian's Cop Car)
02 Toyota Camry (Reliable but a souless steel and plastic hulk)
2011 2SS RS Camaro M6 Factory Hurst Shifter 
Maybe I need to sell some of this crap 
Yet another Bozo with a sawz-all
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