Taxes: A couples' tax filing status is determined by their marital status on the last day of the tax year. This means that if a divorcing couple is divorced by December 31they will not be filing "married, joint" or "married, separate" but will be filing as a single individual or as head of household. How this impacts an individual or couple will be based on many things (i.e., income, losses, deductions, etc.) but a change in tax filing status can often mean a difference of thousands of dollars.
Symbolic/Emotional: There are many steps in the divorce process which are important legally or financially and others that are important on a more emotional level. It is often the case that individuals who have done the hard work to get through the details of their divorce settlement and co-parenting agreement throughout the year would like to put an end to that chapter in their lives and starting the new year as a single individual is one way an individual can make a fresh start with a clean slate for the new year.
Health Insurance Benefits: While not necessarily tied to the end of the year, determining when the divorce is going to be final will have an impact on when a spouse is no longer eligible to remain covered under the health insurance policies of his/her spouse's policies. Knowing how health benefits are tied to marital status and knowing whether and how to extend the duration of health benefits can be useful in a divorce settlement.
Re-financing: Again, this is not tied to the end of year, but when a refinance of property is a key component of a divorce settlement, there are timing issues which we need to be aware of so that the fully signed Marital Settlement Agreement can be provided to the bank to confirm the agreements affecting the property. The banks are not always sure of the exact information they need, but having worked with many, many couples who are refinancing their houses to buy out the other spouse's ownership interest, most banks need the fully signed Marital Settlement Agreement, which is the document that gets filed with the court after agreements have been reached on all custody, support and financial division issues.
Every case is unique, with a wide variety of reasons for both the begining and the end of the marriage. The reasons for finalizing at a certain time are usually limited to taxes, emotions, health benefits or financing options, all of which are related to the separation of the financial entanglements that come with marriage and are sorted through in cases of divorce.