Although we often think of paired samples as being the same person (thing) in a “before” and “after” treatment setting, there are some other important types of paired samples.

One kind are “natural” pairings, such as spouses, siblings, and especially twins. This type of pairing is often used in medical observational research when it is difficult to construct a true experiment. (PennState, 2017)

But even more common are other types of pairing. A more accurate label for this two-sample test is a test for dependent samples. Samples are dependent when there is a relationship of some kind in play which causes the samples to not be independent.

I like this definition from the Minitab blog:

If the values in one sample affect the values in the other sample, then the samples are dependent.Paired samples are not always obvious