I can't speak for the mentioned app (rem), but I built my own app with a similar feature set called ScreenMemory (https://screenmemory.app). Which I have obviously used for an extended period of time (coming up on 7 months I believe).

My main and daily use case it to look back at the previous day - this is helpful for standups, retros, and so on. I skim through my days (sometimes weeks) to pick up on what I was working on - it's incredible how much "untracked" work is performed that you pick up on. Sometimes I forget who exactly I talked to about something, but just knowing the rough date I can usually find something to jog my memory.

Obviously I am biased, but still!

I used to use Google Desktop back when it existed for its viewed web page search feature mostly. It was pretty handy.

Google Desktop was the first thing I thought of when I saw the link. One difference with Desktop though is that nowadays people are doing stuff on more than one device. Synching the data somehow or other would presumably involve the sort of cloud services that this developer is avoiding for privacy reasons.

I build a lot of stuff and forget why I did things the way I did.

You can do things like find the point in history where you fixed a bug and go watch yourself debug and put in the fix.

Pretty wild. It makes a lot more sense once you experience getting the value.

Personally I hope Apple adds the feature natively to the OS at some point. They're plenty experienced to build it themselves over there, but maybe having a reference implementation will encourage them to give it a shot.