What I tend to find is that importance itself is a shifting concept. I've been at the same company for a few years and worked on many projects in different areas. When I start a new project, I tend to forget a lot of the details of the previous one. Then I later get pulled in for information or for a new project that builds on the previous and that old forgotten information becomes valuable again.

Then there's the fact that the keeping of the wiki itself is something I use for rubber ducking and the Feynmann technique.