I think they have several uses.
One was mentioned by
@Lo-Profile right out of the gate: at longer ranges, dots can be more accurate, especially on guns with relatively short sight radii.
Another, as noted by several members, is aging eyes. Dots can be great for folks who are beginning to have trouble seeing irons.
A third is that dots tend to be better in nearly every way
for inexperienced shooters. "Iron sights stratify marksmen", and true mastery can take a very long time. The learning curve with dots is significantly flatter, and even a complete novice can get up to a basic level of competence relatively quickly. (I still maintain that "Dots are more accurate" is, as a general statement, incorrect. The absolute record in Bullseye competition - where dots and irons compete on the same field - is still held by irons.)
I personally am using a dot on a carry gun, primarily for reason #1: I worry about rifle-equipped "mass shooters" who might require precision long-range fire - from a small, concealed handgun. Compared with big plastic "irons", with a 3" sight radius, a dot makes such things much more realistic. Of course, the odds of ever actually
needing to use a gun in that capacity are microscopic, and I certainly would never suggest that everyone who carries needs to worry about the same things I do. The dot gives me peace of mind, though, and so is worth the money, to me.