Through the years I've heard several reasons for the long hose. The two most prevalent is the extra length was used to allow additional fluid in the system for cooling, the second being that the length was used for pulse dampening. My '71 M code Mach1 has AC with a radiator style PS cooler mounted to the compressor and the long pressure hose. I have also seen cars equipped identically to mine without the AC compressor mounted cooler. With the combination of a rubber pressure hose with a foam wrap, I'm not sure how much actual cooling was accomplished. The pulse dampening story.....who knows?!
The long hose was used until 5/1/72 when a pressure hose approximately half the length of the previous hose was used. I've driven with both versions of the hose on my '71 and noticed no difference in how the car drove. I'm presently running the after 5/1/72 pressure hose.
To me, it just looks a little cleaner under the hood without that extra hose mounted to the shock tower brace.