Today I dared to ask a very basic but all time ignored question about the purpose of single row and single column marquee tools in Photoshop.
It will be more appreciated if the answers provided with the visual use cases.
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It's been a while, but before it was possible to create gradients using exclusively CSS it was a handy tool to isolate a 1 px cross-section of a vertical or horizontal gradient to use as a background image.
So if you had the following image as a mockup you could use the single-column marquee tool to grab a 1 x 64 px slice and save it as a lightweight GIF and use CSS to repeat it horizontally.
You might want to read Andrei Herasimchuk's answer to a similar question on Quora.
In short, there's no obvious purpose, but they can be used to create guides by selecting a single line and filling it with a color. This seems a bit redundant now that Photoshop has real guides, but I guess the single pixel line allows you to get a bit more creative about how you use them.