Documenting infrastructure changes over time is useful for spotting trends in your knowledge. It's also helpful when identifying areas that are lacking in investment. Keep track of the major changes to your platform; this table is nice, but you can see there was a large chunk of time I didn't update it.
Things were still changing, but they weren't documented. Going back a year and digging through our ticketing system looking for major changes wouldn't necessarily be trivial, either. Keep your documentation up to date or it's useless!
In addition to a ticketing system, we utilize a CHANGELOG mailling list, where summaries of major operations, development, or policy changes should be sent to, tagged appropriately. We only started doing this in April, though, so populating the missing year is still hard.
On our wiki, I also have brief notes for the content of each release.