The bar pad is what keeps the metal spreader bar from digging into you.... so yes, necessary.
The reason that some Harness manufacturers don't include bars and/or pads as standard is that spreader bars (and pads) come in different widths.
After you get the harness you can pick the bar/pad in the width that is most comfortable for you.
Some harnesses include the emergency knife, some don't.... they generally list it as a feature if it is included. It appears to be optional on a lot
of Dakines, so something you would have to get seperately.
The bar is generally attached to the pad using a combination of cut-outs, velcro and/or webbing depending on the harness manufacturer. Then the bar
is attached to the harness using webbing laced through attachment points at the ends of the spreader bar.
Dakine and a couple of others have tweaked this on some harnesses using various clips/latches. Dakine calls theirs the Power Clip.
If you search on YouTube there are several product demos of Dakine and other harness and how you put them on and take them off.
Hope that helps.
ATB,
Sam
"I never had any friends later on like the ones I had when I was 12 - Jesus, does anyone?" - The Body by Stephen King