Because they let me move stuff faster.
My long-time reader will recall that I move minerals to lowsec by autopiloting them to an adjacent hisec system, then bouncing back and forth with a jump freighter. Jump in, drop off mins, warp to highsec, get more.
Previously I had been able to move two JF loads (73.6 million units) per 10 minute cyno activation. The cyno ended about 30 seconds too early to get in a third load.
Session timers affect this process process as following:
-Jump in. Wait for session change timer. Dock.
-Undock. Wait for session change timer (because hotdrops). Warp.
-Dock in highsec. Undock. Wait for session timer. Jump.
-Jump in the second time. Wait for session change timer. Dock.
-Undock. Wait for session change timer. Warp.
-Dock in highsec.
-Cyno runs out.
After the timer was reduced to 20 seconds, the cyno ended much closer to the time I was ready to jump in a third time -- so close that the JF had to wait for the cyno cooldown to finish so I could relight it. Eventually, by staying on top of things and docking/warping/jumping the second the session change timer finished, I was able to get in three jumps (110.4 million units) per cyno activation, with a few seconds to spare. This means the rate at which I could move minerals to lowsec increased by 50%.
The recent decrease didn't change much, but does give me a comfortable buffer, as shown at left.
If you figure 5 cynos per hour, allowing a few minutes for the JF to get back to highsec after the third jump, I can now move 550 million units of minerals per hour. The average load I move is about 500 million, and I need to move a load about once a week.
So, an hour a week. This is not a huge drain on my time, which is why I haven't bothered seriously looking into mineral compression.