When can a Secondary standard solution be used for standardising something instead of a Primary?
If you are doing pH titrations, and your primary standard is an acid - say - potassium hydrogen phthalate, you can't use that to standardize an acid directly. Instead, you would make a secondary standard of - say - sodium hydroxide, titrating against your primary standard. Then, you are ready to standardize the acid you want to use in your titration.

Secondary standards are also used if the primary standard is expensive. Hydrochloric acid solutions are cheap and stable, but you can't make them by weighing in a solid. So instead, you make an expensive primary standard and a lot of cheap hydrochloric acid of unknown concentrations, and determine the titer of the hydrochloric acid. Then, you can use it as a secondary standard.