Yeah, standalone DACs/amps. You would use these because they give you better sound quality than the phone itself does, and also the amplification makes them able to drive less sensitive headphones, which phones are unsuited for. Now that Sony, HTC, and Samsung all have USB audio in their phones it means I never have to care how good or bad my phones inbuilt sound quality is, I know I'll always have awesome sound quality regardless.
As for why so many, well, no particular reason apart from I wanted to know which I liked best and I'm taking my sweet time about deciding!
Blogs: androidnz.net, blog.clove.co.uk