Speaker phone is probably necessary nowadays since many people only have a smartphone.

With the old phones you could reasonably tilt your head and raise your shoulder to hold the handset in place so you could do something that required two hands while talking/listening, like looking up something in a book or taking notes.

Smartphones are smaller than the old handsets and much flatter. I can pinch mine between my shoulder and head but I've got to raise my should pretty high and do some other contortions to get my head tilted enough making it much more awkward to do anything with my hands. Also that phone is small enough that it is pretty well covered in that position by the side of my face and my shoulder, so I'm not sure the mic could pick up much.