Advantages of second marking In the UK a portion, and in some cases all, work is "second marked" where an independent marker also marks the work. In cases where the 1st and 2nd marker disagree, a 3rd marker may be used. Finally, the entire work of each student over the course of his/her studies is evaluated by an exam board with (sometimes) 2 additional independent markers. These exam board markers tend to only consider cases that are on the border of different degree classifications.

From my understanding of statistics, having all of these different markers will regress marks towards the mean. As I am currently faced with the daunting task of 2nd marking a large stack of off topic papers, I am curious what are the advantages of second marking?
Double marking has many roles, but mostly it is to ensure accuracy and fairness. The main way of achieving this, and avoiding the statistical anomalies alluded to in the comments is to produce an effective marking scheme, so that academics with sufficient background can grade the exam and produce virtually the same grades. Easier said than done.

More details can be found on the Internet, for example, on Swansea University's website.