Because if, for example, you wish to send a "break" on the TX line, yes, you can switch momentarily the alternate function of this pin to GPIO mode, then setting the GPIO to LOW, and after awhile to HIGH and switch back the alternate to UART.
Simply look at this page wiki/doku.php?id=esp8266_gpio_pin_allocations for the pins alternate functions, and use the PIN_FUNC_SELECT() macros to temporarily switch the pin to plain GPIO.