When do I need to use UITableView.insertSections: (and deleteSections:)?

In a plain UITableView, is it necessary for me to explicitly call insertSections to create the number of sections in my table or is it sufficient to just return the number of sections via my numberOfSectionsInTableView data source method? I've been under the assumption that as long as I return the correct number of sections in numberOfSectionsInTableView, UITableView will take care of creating the proper number of sections in the table. It certainly seems to do that when you initially create your table.
But my table data can change over time. Rows and sections can come and go. What I've been doing is simply calling reloadData on my table view whenever the underlying data changes and then letting my numberOfSectionsInTableView and numberOfRowsInSection data source methods report to the table the new number of elements. This appears to work some of the time. But sometimes, after a section has disappeared out of the data source, the table view leaves the header view (that gray bar with a section title) visible and floating above the table - even though I've called reloadData and my numberOfSectionsInTableView method returns the new number of sections. At this point, you can't get rid of the section header view.
What I've discovered is that I must explicitly tell the table that sections have been deleted by calling deleteSections:withRowAnimation:. The converse holds true. If my data source has added new sections, then I must explicitly call insertSections:withRowAnimation:.
What I don't understand: why do I have to use insertSections to add new sections as opposed to initially, when I created the table, I did not have to use that call. I simply let my numberOfSectionsInTableView inform the table view how many sections there were?
This is an issue for me because of the way that my data changes. I do not know what sections in my data were deleted. I only know that something changed in the data and I need to repopulate the table.

have you tried calling "reloadData" on the tableView?

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When do I need to use UITableView.insertSections: (and deleteSections:)?

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