When I looked up commentary on this psalm, I came across discussions of when this psalm was written; i.e., what particular national crisis had happened, which foreign power had overtaken them. And I could see that. Zion--the name for the whole country.
Yet, we can read this psalm as a model for prayer for more particular circumstances, including a personal crisis. The psalmist's argument (rationale) for asking God for help is that God has provide help before and can provide help again.
A second yet--Back to the national, God has and can provide help for a nation.