You rely on ONE headphone jack for all your audio. Often you need to switch between headphones, speakers, internal, etc. This leaves you with plugging and unplugging, and plugs that are left dangling, sliding off the table, or on the floor. iMac and other all-in-one users have the added frustration of feeling around for the correct ports in the back (ever mix up the headphone and microphone jack?).
Sure you can search for some audio device but they are too limited (one mic and one headphone jack) and/or too complex (extra do-hickeys and outdated cables with stereo-type plugs). Few actually offer switching functionality and those that do only allow you to switch INPUTS, not outputs.

A USB audio adapter that offers more ports (audio jacks + usb's) and a physical switch to select audio outputs. Now keep everything plugged in, and slide the switch between headphones, earphones, speakers, mic, or off for internal/no audio. Get this functionality from ANY USB PORT. Added USB port(s) means you can still get USB connectivity out of the port. No limits and nothing extra, keep it simple.

(Note: pictures represent most basic function for an iMac-specific application. Design may take any universal form)