If you're senior or have opinions about things, you know the feeling of falling into a rabbit hole of stuff you want to fix when you look at certain parts of your system. "I was going to rewrite this 3 months ago", "oh wait this part sucks too", "wtf is this class even for", etc.
Before coding agents, I'd have to weigh fixing these against my official work commitments, often getting shot down when I tried to get it prioritized or tsk tsked for delaying official projects to make code nicer. Now, to a much greater extent, I can just fix the things. The agents aren't perfect and the process isn't anything like hands off, but it's enough of a speedup that I can fit it in alongside my other work without having to get approval for it or try (and fail) to get it formally prioritized.
Not quite an oh shit moment, but having the end result of those rabbit holes be that the problems are fixed is pretty cool, and far preferable to what was often the case before ("we'll put in a ticket and prioritize it during the quality sprint!").
edit to add another:
I've personally never been a big fan of preplanning architecture at a code level. It makes a lot of sense at the system and data modeling levels, but code is both easy to get wrong if you're whiteboarding it before you write it and relatively easy (compared to system design and data modeling) to fix when that happens. If it's just me on a project, I'll happily start bashing it out with a vague idea in mind and evolve the design as I go, knowing that I'll probably throw a way a bunch of what I write at first. I know I do good work that way, and I'm not wasting a bunch of up front time on a design I'm likely to throw out later. It's hard to work that way on a team, especially as a lead, for obvious reasons. Coding agents fit really well for that work style. They'll cheerfully write dueling prototypes of my code architecture ideas so I can see which one I hate and which one I like without talking about hypotheticals and abstractions on a whiteboard. They never get mad at me for changing my mind, wasting their time, or throwing away their work. That's pretty cool. I can have a quick, cheap answer to "what would this look like if I got rid of class X and split its responsibilities between Y and Z?", and I don't have to feel guilty for wasting my time or my teammates time if the answer is "oh man that sucks, what a terrible idea."