What is the reason to open the low side of tank transmitter tapping to the atmosphere
Because the pressure on the surface of the open tank is just atmosphere pressure, so we leave the low side in the atmosphere.
To eliminate the atmospheric pressure
One caution with respect to “tanks open to the atmosphere”.
Some applications use vents to contain highly volatile liquid vapors. The vents allow a build-up of pressure to a couple inches of water column, then release the pressure and let it build until the pressure reaches setpoint again.
This type of situation requires a piped connection from the vapor space to the transmitter, because the vapor pressure build-up will appear a level change if not compensated with a low connection to the DP transmitter.