It makes it so much easier to pick them up again in the future when enough time has passed that I've forgotten almost everything about them.
I have a couple of android projects that are four years old. I have the architecture documented, my notes (to self) about some important details that I thought I was liable to forget, a raft of tests. Now I can't even get it to load inside the new version of Android Studio or to build it. There's a ton of indirection between different components spread over properties, xml, kotlin but what makes it worse is that any attempt to upgrade is a delicate dance between different versions and working one's ways around deprecated APIs. It isn't just the mobile ecosystem.
Every time I've believed this, I came back to broken CI.
Depending on your language ecosystem, your CI workflows and build deps stand a greater chance to break than your code.
I have a rule that any commit which changes the implementation has to include the documentation update at the same time.
Most of these documentation updates are a sentence or two, or maybe a paragraph. The overhead of incremental documentation updates like that is tiny enough that I don't really think about assigning extra time for them.