Matt Webb has written a lovely piece about 'er', comparing the meta-linguistics meaning of it when uttered verbally (which he pins down pretty accurately), with the 'undo' key on the keyboard:
From this perspective, how could Undo on my computer be different? "Undo" is functional "er" in a way. To say "er" is to safely experiment with a way forward. If my computer had pervasive "er", I'd try out different routes forwards more playfully. Say, in Photoshop, I'd try one sequence of actions, then if I was happy I'd commit to it and if I wasn't I'd, er, try another.