Generally speaking, the BNC connectors are better impedance matched than the M-type (SO-239) connectors. And this may affect more on V/UHF frequencies.
So the rear M-type connector is for the HF and the front BNC connector is for above 6m as the default settings of FT-817/818, I suppose.
However, such a QRP RIG, I personally do not care for the impedance matching but I select connectors for my convenience. hi
// HIRO, JJ1FXF
HIRO, JJ1FXF