It’s not about the low level details it’s what you discover durning the process and building a mental model of the project and the style the code was written in.

Also, memory isn’t just about unprompted recall. If in 8 months you’re looking for how the project generates PDF’s or whatever your not starting from scratch. Sure a full text search will probably bring up a bunch of files but skimming them a second time should look familiar. You may even remember that different sections of code are using two different 3rd party tools because that’s the kind of thing you notice an a first read but might otherwise cause all kinds of shenanigans.

Of course reading takes time, but it’s kind of like negative technical debt. Your prepping so anything else you want to do becomes faster. Asking coworkers about code before they jump ship can clarify stuff in minutes that might take days once they left. Even better you often learn something generally useful on other projects.