I started diving last year, and like most new divers, I went through gas like Darth Vader. I found that my gas consumption being higher than my buddies, I was causing them to shorten their dives to come up with me. As I was doing a lot of drop in dives with my local LDS, they want ppl to dive within their certification, which means, I could not come up by myself, even through I was packing redundant air. Getting my solo cert, allowed me to dive with my friends in group, and yet, allow me to come up solo, when my air usage dictates, leaving my buddies to their longer dives. By diving within my certification, I am allowed to Solo on the group dives without having to worry about the shop having liability issues. My DM friends do not have to worry about me.
Edit: Since my Solo cert, I have now done 16 completely solo dives, including 2 night dives, these are not part of the group dives.