After updating the firmware of the IC-7300, the DF of the station I was receiving became slightly different between my friends, so I used the FreqCal mode of WSJT-X to calibrate it.

Probably the value of the reference frequency adjustment has changed.

After this calibration, there is no more frequency discrepancy between me and my friends who are using external reference signal with IC-7610 even on 6m band.

If you don't mind a slight shift in frequency, you don't need to adjust it, since the difference is only a few Hertz. However, if you are concerned about the slight difference in frequency when exchanging information with your friends or QRV information in a group, this is a good option.

For more information on how to calibrate, please read 13.1 in the user guide.