How to choose UART maximum output speed?
The GPIO speed setting just configures the drive strength or the ability how much the pin can source and sink current. More current means faster signal transitions to heavier capacitive loads.

1) For example driving a FET gate which is highly capacitive and it needs to spend minimum time being halfway on

2) Somebody has chosen these defaults often just because you need to have some basic default setting that mostly work. Using say 3 megabaud with long wires for debug UART might be safer to have high drive ability, but blinking a LED on GPIO once per second does not matter which setting is used.

3) Of course they can communicate if the baud rate used is slow enough that signal slew rate does not contribute to the error much. Weakest drive strength can work to about 2 MHz maybe, depends on which MCU is used.