I was looking in a residential service panel today. 150A, 30 circuits, 15
per side.

For the circuits that run down the right side of the panel there is a bus
bar that holds the ground wires and a separate bus bar for the neutrals.
The ground wire to the water meter was attached to the neutral bar.

For the circuits that run down the left side of the panel there is a single
bus bar and the grounds and neutrals from each circuit are paired up under
each screw.

A strap connects the neutral bus bar on the right side to the
neutral/ground bus bar on the left side.

Why are there 2 separate bus bars on one side but a combination bus bar on
the other? Aren't they all electrically at the same point? If so, why
separate them on one side?