Choosing to limit a setting to only one species (say, Humans) has some advantages. The biggest is that the writer doesn’t have to develop multiple species but can still explore different cultures. On the other hand, the choice can limit variety somewhat and reduce the effect of cultural exploration. With multiple species, certain cultural traits can be exaggerated in a way that would be comical or grating if the species was Human—imagine the Vulcans, Klingons, or Hutts as Humans.
That said, even in a Humans Only setting, it is possible to have variants. If the technology is advanced enough or the magic is capable, genetic modification can occur (SJGames’ Transhuman Space). Or there’s parallel evolution (Jack Williamson’s Darker Than You Think). Biomodification (after birth; biological version of cybernetics) or cybernetics could effectively create different (mostly) Human sub-species. Mages and/or psis could even be genetically different enough to be considered a variant species. Or super powered mutants (Marvel, I’m looking at you).
Our folklore and legends are filled with Human variant species—such as Elves, Dwarves, Giants, Brownies, Blemmyae, and Cyclopses—as well as human-based hybrids—including Centaurs, Cynocephali, and Satyrs.
This sort of expansion of species, while technically being limited to one, has the benefit of providing multiple species to work with but keeping them all humanoid. There are similar drawbacks to multi-species settings in terms of culture (if the variants create their own cultures). The question of cross-breeding also arises and can be a thorny area. A final drawback, that could also be a benefit for some, is that many of the traditional Human variants have been used by more than a few writers in the last century or two. This has made them familiar (a potential benefit) but also stereotyped and cliché (drawback, perhaps).