The rear panel jack usually requires a high drive level. Radios are designed so that a very weak signal on this pin will not be transmitted. This allows you to keep the TNC cable plugged into the radio when operating voice. If there are small noise fluctuations on the cable, they will not be transmitted with the voice signal. Probably no narrow receive filters.
Most digital modes use a narrower bandwidth than voice.
With voice filters, there is normally a lot of extra noise received with the digital signal. To get rid of some of the noise, you can use passband tuning or add an external DSP audio filter. No need to worry about room noise getting into the transmitted signal. TNC output drive level and radio mic gain control are not applicable. Narrow receive filters.