You can merge together adjacent cells to create one cell in a table. This is typically used for formatting, to provide a larger space to center a title over a group of cells. It typically isn't needed in Numbers, since sheets can contain multiple tables. So each group of data can be in its own table. However, to create spreadsheets that are compatible with Excel and other apps, using merged cells can be useful.
Want to know more about how to use Numbers on your Mac? Check out this MacMost course!
Where can I take a course to learn Numbers?
Rich: Look in the right sidebar here to see links to all of my courses, including one on Numbers.