This attribute provides a summary of the table's purpose and structure for user agents rendering to non-visual media such as speech and Braille. This attribute offers advisory information about the element for which it is set. Visual user agents allow sighted people to quickly grasp the structure of the table from the headings as well as the caption. A consequence of this is that captions will often be inadequate as a summary of the purpose and structure of the table from the perspective of people relying on non-visual user agents. This is especially important for tables without captions.
Examples below illustrate the use of the summary attribute.
Summaries are especially useful for non-visual readers. If we assume a DOM is available, then at least 3 is easily available. Or does the content of summary describe what the structure does not reveal. In this example, there are most likely 9 columns each with an expense or income heading.