Undo
I wrote recently about static and dynamic information in The Banished Vault and how those are used to make a challenging game consistently difficult. One aspect of the difficulty that goes unmentioned there is, of course, undoing.
The Banished Vault does not allow the player to undo a turn. The player can restart a system if they like, and can configure a journey to allow infinite, one, or no restarts. Does this make the game harder, by not allowing small mistakes to be corrected? Yes. But that is not the primary reason the player is not allowed to undo a turn or move.
I'll quickly lay out all the other small downstream effects before I reach my main point. Firstly, restarting a system does give 'free information' to the player, in that they can get hidden information, restart the system, and play with that information in mind. This is a valid way to play the game, because the game allows for restarting. The cost to this is the player's time. A player's time is, in an abstract sense, a resource that the designer can play with like any other. Players have different tolerances to how their time is spent, but in this case they can choose to spend it to get that information.
Not having an undo also provides important tension to the player's decision making. Every click is important, every decision is (mostly) final. Of course, the player can also pay a cost to undo a click, and that cost is their time to restart a system. This cost scales as their time in a system lengthens, which is a nice benefit. As a designer I am not fussed with a turn one or two mistake and will let the player have that one, as it were, on the house. When the player is twenty-five, thirty turns deep into a system, the time cost to restart is rightfully high. This reinforces the themes of the game and squarely pulls them into the emotional goals I have for them.
All those are good reasons I feel, but not quite exactly the true purpose of no undo. Those goals above can be recreated with other methods, using resources in-game or some other mechanic. I do not believe there's another effective way to achieve the real goal: avoiding tedium.
A single-player, turn based, heavy on numbers game like The Banished Vault can very quickly become tedious. At many points in development it was tedious! With only specific sets of information that is hidden from the player, it can be easy to treat a solar system like a puzzle with one ideal solution, and then just performing the set actions to achieve that solution. Some players do treat it that way, and that's fine, but my goal is for the majority of players to do the best they can, make some mistakes, and keep moving forward.
An undo system would undercut that immediately. For both the player's actions, revealing new information, or fudging hazard rolls, an undo system allows the player to think...
I could probably do that better, let's if there is a better way.
...and that's what I don't want. It's important for the player to make mistakes, but I don't want the player to start optimizing each solar system because they can. An undo system in a strategy / tactics game is implicitly telling the player "there is an ideal solution, and the game expects you to find it." I, as the designer, do not want the player to find the ideal solution! The size of the solar systems, the distribution of resources and hazards, the many different paths the player can take to move forward in the game, there are too many possible routes in the maze. If the game implicitly tells the player it expects them to always do it the best, the player will spend way more time than they need to looking for marginal improvements.
So ultimately that's where things ended up with the undo system. All the tension and mistakes are good! But really I just want the player to not worry about having to fully explore every last fork in the maze and just move on with an imperfect solution.