[OnlyBodyWeight]

What is the use for these?
http://www.ebay.com/itm/3FT-3-5mm-1-8-Male-to-Male-Stereo-Audio-Jack-Cable-for-iPod-MP3-MP4-DVD-Green-/290765541701?pt=US_Audio_Cables_Adapters&hash=item43b2fa1145
 
I am going to buy this so I can hook my phone/iPod into an external speaker.
What other uses does this thing have?

 

[D3Seeker]

That pretty much. Hook up your (*insert PMP/DAP/audio device of choice here) to speaker or sound system. I guess there are also the case of line in/out between recorders and what not. They are useful when the device you want to hook up doesn't have a proper digital hookup, or mismatch between such devices.