This is a Summary Score, which summarises the results of many individual tests.
What does it summarise?
An overall summary score for the website. Note this is not the average of the other Summary Scores, as it includes some additional factors not covered by them. At a glance, this is essentially an answer to ‘how good is this website’.
Why is it important?
It is often useful to have an at-a-glance metric which rates the overall quality of a website as a single number. Although you should always inspect individual tests to gain a full picture, if you have 10 websites, sorting them by Overall Score and focusing on the bottom 3 first might be a good way of prioritising the most significant changes. Similarly, if one site scores 3/10 and another 9/10, it is fair to say one is better than the other.
See also Perfectionist fallacy.