Why do stairs have overhangs?

Most stairs (at least here in the US) have an overhang or 'nosing' where the edge of the stair protrudes over the riser:

nosing

I've heard different reasons for why the nosing exists, but I've not been able to find a definitive source. Reasons I've heard are:

  • It provides more space for feet (I'm not sure this is actually true, since it comes at the cost of potentially tripping on the overhang)
  • It makes the stair more visible
  • It somehow improves safety
  • It creates clearance for toes and heels biomechanically

Can anyone show or point to a source which definitively shows why having the overhang is better than not having it?

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