For the same reason that RS485 doesn't "need" ground but it is usually provided: to keep the common mode voltage under control.
Put another way, RS485 needs ground nearly always (the exception is transformer isolated connections).
most RS485 chips only allow about 7 to 10V of ground difference before they are out of spec, so operating without
some means of keeping the grounds close in voltage is not guaranteed to work (as capacitively coupled stray
voltages in everyday environments can be 50V or more)