We were always taught to have the ADI set to level on the ground. This means when at night or in cloud, if you have to make an EOL purely on instruments, then just before the little bump at the bottom, you will know if your skids/wheels are level for running on. In the Navy as a student we practised EOLs at night on instruments and I was surprised to discover I did them better using instruments than using my Mk 1 eyeballs by day! Fortunately my eyeballs did improve!
By having the ADI on a fixed datum, it meant that you had a fixed angle for whatever speed you wanted which made instrument flying much easier. For example a Wessex did 90 knots at 4.5 degrees I think.
