What type of letter is Pierre Deligne talking about and why did he write a letter to other mathematicians? Today I watched a YouTube video interview of Pierre Deligne. I have some confusion about a statement given by Pierre (video timing: 56:33 to 58:20). Here is the outline of the statement in the video:

Interviewer: Would you like to write letters to other mathematicians?
Pierre Deligne: Writing a paper takes a lot of time. Writing it is
very useful, to have everything put together in a correct way, and one
learns a lot doing so, but it’s also somewhat painful. So in the
beginning of forming ideas, I find it very convenient to write a
letter. I send it, but often it is really a letter to myself. Because
I don’t have to dwell on things the recipient knows about, some
shortcuts will be all right. Sometimes the letter, or a copy of it,
will stay in a drawer for some years, but it preserves ideas and when
I eventually write a paper, it serves as a blueprint.

What type of letter is he talking about and why does he write a letter to other mathematicians?
I mean, there are many different types of letters out there. I mean, some love letters, friends letters, etc.
I just did something like this last month...
Well, with modern technology available, I didn't write a letter; I wrote up some notes, scanned them in, and attached them to an e-mail.  Keep in mind, though, that Prof. Deligne started his career in the days before e-mail, and telephone calls (from rotary dial phones attached to landlines) were expensive (like a dollar a minute.)
I had an alternate approach to proving a theorem in a paper I had read and thought perhaps the approach could also generalize beyond their theorem.  I wrote up a sketch of the alternate proof (for which some details I have not actually proved) in 3 pages of notes.  I scanned them, and attached them to an email to the authors that read something like &quot;Dear X and Y, I hope you are well.  We talked about this at Online Conference Z and I'm attaching some notes.  I'm happy to meet by Zoom and talk.  Best, -me&quot;
I didn't write up a paper because it would take 10 pages and at least a couple weeks of work to give precise definitions of everything, and any paper would only be publishable in a fairly low-tier journal (since the result isn't new and the approach also isn't new, having been applied to other problems before), which at this point in my career doesn't count for anything.