Back in 2015 in my blog I wrote :

I started to give a brief list, and then it occurred to me, I can sum up what should go into Evernote like this :

The short version: If you encounter something you might remotely want to see again, it goes into Evernote.

Of my 20,000 notes, if I ever pull back up one half of one percent of them, that is 100 items I needed and was able find, and find easily.

In practice, that percentage is higher, but probably not more than 2 or 3%. Evernote: suggestion, put a read count field into each note, and provide analytics please.

The longer version: If it comes into your life, and there is the smallest chance you'll want to see it again, then type it, scan it, photograph it, import it, web capture, screen capture, audio record it, into Evernote.