On Tue, 18 Feb 2014 09:48:17 +0000 (UTC), Judy Miller
wrote:

It always seems I have the wrong diameter headphone jack when I need one.

They started with big ones, 100 years ago. 1/4" As things have gotten
smaller, the big plugs were too big for the little devices.

Since adapters exist, why do they make headphone jacks different
diameters anyway?

I have loads of adapters, but only for temporary use until I get the
right product. Adapters are a pain -- they stick out -- and a little
cell phone or Sandisk MP3 player won't have the power to drive the 2 or
2.5" speakers in a large set of headphones. Even if it did have the
power, it would run the battery down too fast.

Is there a functional difference?

No, except that some headsets need a microphone, and others are
monoaural or stereo, maybe with no microphone. For talking on the
phone, there could be two earpieces but they would have the same sound,
no stereo.