In this question, people mention that
biber to work. However, if
biblatex has the option
backend=biber, doesn't that mean that using
biber is optional rather than mandatory?
Up until several (five or six?!) years ago, there was an age when the
biblatex package was not yet as mature as it is now and the
biber program was, ahem, less than perfectly stable and reliable. Understandably, the default back-end program (to perform sorting) was BibTeX. If one felt a bit adventurous and wanted to use
biber, one could (and had to) specify
backend=biber to override the default.
At some point, however,
biber became the default back-end for
biblatex, mainly because (a) it's much more powerful and versatile than BibTeX -- which is, after all, what
biber was supposed to be all along -- and (b)
biber eventually became sufficiently stable to be usable outside of, to put it delicately, experimental settings. Ever since this switch was made, it has technically speaking no longer been necessary to specify
backend=biber if the intent is to use
biber as the back-end. Conversely, if one nowadays really means to use BibTeX, one must specify
backend=bibtex explicitly. A separate matter: there are by now some biblatex bibliography styles that require
biber, i.e., they won't operate properly if BiBTeX is used.
As you probably know, though, old habits -- whether good or bad! -- are slow to die. Thus, many old hands, and quite few younger ones too, still write
backend=biber even though it's no longer strictly necessary to do so if the plan is to use the program
As mentioned by Mico,
biber is the default choice for
backend but one can also give
bibtex. As already detailed, that choice was made as Biber offers more features than BibTeX. It is worth noting, however, that
biblatex continues to allow the use of BibTeX as a back-end (there are no plans to remove the support). One factor in this is that
biblatex was originally written to use BibTeX and that for quite a large number of 'simple' use cases (large parts of natural sciences, for example) the outcomes are equivalent to those for Biber.