Often,, a letter can provide a bit more data than what you might expect.
In recent times,, they are adding the month of manufacture, AND the month of shipping. Plus,, they say where it was shipped to.
Even a plain jane model may have gone to a place or a person that may have significance to others.
It's obvious when a gun may be a rarer variation or low production, or even to a subscriber.
Boxes give model info,, but often a gun is separated from the box.
A letter verifies how a gun, with a specific serial number was built by the factory. When someone asks a question; "Did Ruger mark the gun with ,,,,,,,,,,?" we can use a letter to verify how it was built.
But,, yes,, there are hundreds of thousands of guns out there without any significant value other than as a shooter, that a letter wouldn't help anything at all.
Heck, people throw away boxes, manuals, paperwork etc. Those folks never bother with a letter.
Personally, I like having letters on my Rugers,, even if they are not special in any way. Why? As noted above,, one day,, if the expense of a letter increases,, and we wanted one, many will whine about paying for it.
At the last ROCS/OGCA show,, a gentleman came to see me specifically. Last year,, he'd mentioned a gun he had. I strongly advised him to get the letter,, as,, if it lettered as he described,, it'd increase the value. Well,,, it lettered,, AND,, more importantly,, his gun helped chad (chet15) update details on that model for the RENE. His gun was made a month prior to known knowledge.
Collectors like such details.