History: The IBM 3270 series terminals, which predated the PC and with which all the original IBM PC developers were familiar, had separate "Return" (carriage return) and "Enter" (submit form) keys. This architecture minimized precious CPU interrupts. You would fill out an entire form in the terminal, with multiple fields and multiple lines on the peripheral device, then send the whole form to the CPU in one action. This was important in an architecture originally designed for batch processing, and which referred interactive support as the "Time-Sharing Option."
That's a long way from the Unix model, in which every keystroke generated an interrupt, or the PC paradigm, where every key-down and key-up action generated an interrupt.
On my IBM PC keyboard, the word "Enter" is above a "return" symbol. So for sites like Facebook, which require a shift for the "return" function, the non-shift action is the upper marking and the shift action is the lower marking!.