Why make a Joint Will?
The most common situation for a couple to be making a joint will in is of course if they are married and they want to provide for each other and when both are dead for their children or other relatives. As the overall beneficiaries are the same in both wills the terms of the wills are the same with just the names swapped over in each will.
Why make a new Will if you marry or remarry?
It is worth remembering that if you make a will and then get married or remarried this cancels a previous Will unless it specifically states otherwise. This is because the law presumes that you would want to provide for your new spouse and if you do not make a new will the rules that apply when there is no will make sure that your spouse is provided for. It is a good idea once you are married for the two of you to discuss making a will and decide to whom you would like to leave your joint estates. For most couples making a will becomes a high priority after they start having children as they start to think about guardians for their children if they both died.
What happens to your Will if you get divorced?
Similarly in England, Wales and Northern Ireland, your Will is partially cancelled automatically if you get divorced or your civil partnership is annulled after you have made it. Any gifts in favour of your spouse will be cancelled (unless the Will states otherwise) as will any appointment of them as an executor. Effectively your Will would be read as if they had already died.
As you can see it is really important that you think about making a Will again if there are major changes to your personal circumstances.