Anytime you need to change something you have to understand it first. I’ve not seen any evidence that this can be done without reading the code first.

I’ve personally never worked in an environment where code was only thrown out and replaced instead of modified.

Are you counting reading code while you’re developing it? That would sound strange to me. If you’re talking maintenance by someone other than the original developer the pattern I’ve seen is minimal engagement with the code and an almost immediate press to replace it. I’d guess you’re talking about an idealized world.