Why do show societies favour running their open shows on a group system--or not, as the case may be?

It means you don't have so many having to stay to the end only to not get anything.

On the group system there will be 4 dogs placed in each group, so 28 awards, plus BIS and RBIS.

On non group al those BOB's are vying for just two top spots of BIS and RBIS.

Ditto for puppy awards

Non group works well if there are very few breeds scheduled so groups would be very small or unbalanced.  Say in an area with mostly gundogs and no terriers.