Whats diff between. Ht bypass vs using a passive connection-unity gain

When engaged, the HT bypass simply "passes-through" exactly what is received at the input. When HT bypass is engaged, the pre-amps volume control has no effect on the signal that is being passed-through.

A passive output is basically a volume attenuator. At full volume, it will pass-through exactly what is received at the input. However, the volume control will affect the signal, so anything less than full volume, the output will be attenuated (less than the input).

A passive output is not unity gain, except at full volume. At anything other than full volume it has negative gain (volume attenuation).

Convenience.  The idea is that with an HT bypass you never have to re-set the preamp volume to unity.

No matter how you were listening to music, once you switch to HT bypass the gain through the preamp is always the same.

Could you mark a spot on a conventional preamp, and use it all the time for L/R outputs of a 5.1 processor?  Sure, but it's inconvenient. :)

Technically HT bypass does not have to be unity gain, it just has to be exactly the same gain every time it is switched to. The HT processor will handle any differences in the speaker configuration settings.