Freezing the header allows you to view any row while the header remains visible. This makes it much easier to identify which column represents what, especially on a table with many columns.
In this document, the header is frozen. Even though the scroll bar is nearly at the bottom, the column labels are clearly visible.
To freeze the column labels, turn on CSV mode, then enable CSV > Headings (Freeze Panes) > Column Heading 1. If you have 2 or more header rows, you can select Column Heading 2, 3, or 4. If there are identifiers for each row, you can also choose a Row Heading from the same menu. Enable Read Only in Column Headings to prevent changes to the headings.
The heading information is used in CSV tools, such as Join CSV, to ensure the result table retains the original headings.