Absolutely agree with Barmaid. The point is that an experienced person can spot difficulties which the lay person cannot begin to see and will word the document to avoid those. Don't be fooled by people who say that their granny wrote hers on a form she bought and there were no problems.They aren't the ones who got caught out. They might not take out insurance because everything will be all right and it is, but that's no reason not to have it. And the risks in will writing are greater than most insured risks and a lot less obvious to the ordinary person.
As an instance, my own father wrote his own will, copying the formal wording on a will form. When he died I had to find the witnesses to it, because the old form of words he'd used didn't specify that they'd signed in each other's presence as well as in the presence of him.Modern wording is different in that regard. The probate registry wouldn't accept the will without the witnesses ,or one of them, giving proof of the fact. Now that was someone doing the equivalent of using a will form, and that's without any legal or interpretation difficulty there might have been in the text !