And why exactly does a single player game have to be balanced? What is there to balance against?
In games design? Several things! Lots. Loads.
Let's look at "interesting choices".
You make a game that has a list of abilities to choose from, let's say there's ten, and lets call them perks since that seems to be the current nomencalture for this kind thing nowadays.
So, as the game's designer, you have a list of ten perks. The player will be able to select three during the course of their playthrough. As it so happens, in playtesting, you realised that two of those perks are super goddam awesome sweet, and the rest offer varied other moderate benefits.
The games released. Within two days, people are talking about the game and it becomes obvious that you always pick the Super Awesome perk first, then the Goddam Awesome perk, then whatever.
You ever play XCOM, the recent one from 2K? Two words: sniper, Squadsight.
Now, there's a quote from a podcast I listen to, 3 Moves Ahead. It's a podcast that focuses on strategy games and the hosts of the show had a conversation about what defines a strategy game. The shortest version of the answer that they could arrive at was, paraphrasing here: "A sequence of interesting choices."
Their point being, the intrinsic part of what engages the player is the act of considering options, selecting one, then dealing with the results. To have a choice be interesting, there needs to be balance. If one is obviously more powerful than the second, that's not an interesting choice. That's deciding whether to apply a handicap to yourself for no good reason.
And while D:OS isn't 100% a strategy game, it certainly has (for example, character build) strategy elements, just as sitting around a Dungeons & Dragons table has (for example, improvisational) strategy elements, just as Spelunky has (for example, split-second reaction) strategy elements, just as any number of other videogames, single-player or otherwise, do. The 'interesting choice' observation is one of the (many) things at the forefront of the mind of any games designer worth their salt. And, to reiterate, you can't have interesting choices without first having a certain amount of balance across the options available to you.
EDIT: Gosh, I typed a lot. Oops.
Short version, smartly applied and considered balance makes games more gooder.
And I'm going to repeat what I put in my previous post:
There's a specific reason that the Cyseal area / first ten levels of this games were so good upon release. There's your main reason why even a single-player game benefits from balance passes.
