I batch cook and freeze meals, and some of them look similar (sauce and chicken vs sauce and pork) and I want to eat the older stuff first. There are also some products that are recommended to be disposed of within X days of opening, which fall well before their best by date.

Unopened, a jar of pasta sauce is good basically indefinitely, but as soon as you actually open the jar the clock starts ticking. We don't make enough pasta at a time to use a full jar, (and in fact will usually use a small fraction of the jar) so I write the date that I opened the jar on the lid to plan its use a little better. "Hey, better find a use for this sauce, it's going to go bad eventually."

When freezing something you made or something unfrozen you won't be able to finish before its expiration date, it's good to write the date on it as well as what it is when it's not immediately obvious (for example... frozen duck fat and coconut oil look pretty much the same, and they don't smell anything when frozen).

Food that's not prepackaged. e.g., I recently threw out a container of eggs that had been in my freezer for about two years because my hens were laying so much faster than we could consume, that we had dozens of extras.

I also label things like the date I install a new HVAC filter, or how much to cut off on a piece of lumber, etc.

This is handy if you're doing things like separating a package into portions for your fridge for near term use and freezer for long term storage. Such as the large packages from Costco/Sam's Club.

When I open milk, I write the date on the cap to help keep track of how long it'll remain good.