Volunteering More Righteous Works during Hajj (2:158)

What does it mean when it says “volunteers more righteous works“?

Hajj Pilgrimage

2:158 The knolls of Safa and Marwah are among the rites decreed by GOD. Anyone who observes Hajj or `Umrah commits no error by traversing the distance between them. If one volunteers more righteous works, then GOD is Appreciative, Omniscient.

This statement is in general regarding doing more righteous works at Hajj and not just in regards to the traversing between the knolls of Safa and Marwah.

To do more righteous work could constitute praying more, or doing more charity during Hajj

It is interesting that God did not put the history of the reason we traverse the knolls of Safa and Marwah in the Quran.