On a number of vessels it was possible to increase the signal level to the receiver by adjusting the tuning on the powered off transmitter. This was useful when attempting to copy wx or tfc list from a distant mf cs. The larger transmit aerial coupling signal into the rx aerial. Therefore it would be beneficial to use a working frequency close to the cs frequency, (if you had one).
Off topic, on my first ship the hf tx was only equipped with 6,8,12 and 16Mhz.