Ah, let's see…
Right now, no current use for my 3GS, my 4 and my 4s. I'm waiting on ideas to use them
Both my iPhone 5 and my wife's iPhone 5 are serving as occasional speakers (through AirFoil Satellite) for my Mac Pro, as well as 'stereo' sound for the Westminister Chimes app (on the quarter hour).
My 6+ is intended to be the music streamer on my bicycle, if I ever get back on to it. In the meantime it's for anything I need it to be for.
My Pixel 3a XL is for anything I need it for, and anything an Android can do that my iPhones won't necessarily be able to do easily.
My 6s+ is my secondary line and goes with me peroiodically.
My 11 Pro Max is my primary phone. <-- Oh, but it's on the iOS 26.1 dev beta, so maybe not what you were asking.