I am confused about DPI. A 24MP print at~A2 size is like ~240DPI. What's the benefit of having ~20x that resolution? I suppose it will be nice to have the ability to print a 24mp 5x7, but it seems a bit much. Are those resolution numbers real?
This is a very common issue, but you are confusing
pixels per inch (PPI or ppi) with
dots per inch (DPI or dpi).
A 24 MP camera captures 4000 x 6000 pixels, so if you print that borderless to fill an A2 sheet, you have somewhere around 240 pixels per inch. Each of those pixels can represent almost continuous tones, i.e., a huge range of colors, millions of them anyway.
But the printer only has 4, 5, 6, 8, 10, 11, however-many colors of ink with which to
simulate those millions of colors. Suppose my image has a red pixel. On my R280, or an XP-960, there is no red ink. To simulate red, the printer can spray some area with magenta ink and another nearby area with yellow ink. Maybe it's a dark red, in which case the printer can spray some magenta ink, some yellow ink, and some black ink. Or maybe it's a light red, in which case the printer can spray some magenta ink and some yellow ink and leave some paper uncovered with ink i.e. white. So all of these inkjet photo printers are simulating the ability to print millions of colors by actually printing different amounts of a far smaller number of colors of ink.
You can read about some of these concepts--what's being described is not exactly the same as this--here:
https://en.wikipedia.org/wiki/Dither
Now getting back to these printers: the only way some area sprayed with magenta ink plus some area sprayed with yellow ink looks like red to our eyes is if the area are very small and mixed in with each other.
This is an oversimplification, but the dots per inch basically tell you about how fine a mix of ink colors the printer can use to simulate the much larger number of colors in the pixels. So if we have an image that's 240 pixels per inch, and print it on an XP-960 or R280 or some other printer that can print 5760x1440 dots per inch, then we have (sort-of!) a grid of 24x6 dot spaces that we can fill or not fill with various colors of ink to simulate the pixel's color. Consider a gray pixel: these printers have no gray ink. They simulate it by printing fewer or more but not all of the dot-spaces with black ink. With that 24x6 dot grid, there are 144 spaces. That means the printer can print anywhere from 0 to 144 black dots to simulate the range from white (0 dots of black ink) through increasingly darker shades of gray (1 to 143 dots of black ink) to full black (144 dots of black ink). So these printers at that resolution sort-of theoretically can print 145 different tones from white to black.
In reality these printers' function is more complicated than that. But the fundamental truth is that how smoothly and fully any given printer can simulate the full range of colors we can see is due in substantial part (not entirely!) to how many / what colors of ink it has and how finely it can print a grid of ink dots in the space allocated to print each pixel.
