Originally Posted by meme
Its the first free font I came across for doing small caps and I liked the look of it compared to a couple others I tried. Whether its the best font choice I can't say
I don't believe calibri is free to use in EPUBs but could be wrong.
Edit: Of course, the font is only used for the chapter headings. The font for the rest of the text is up to the reader to set on their device
Calibri is NOT free to use in ePUBs unless obfuscated, and even then, I think it's a delicate line.