It is not hard to understand why the layman might feel that leaving their assets to their heirs is a simple and straightforward matter that anyone can undertake on their own. You state your wishes on paper, appoint someone to carry them out, and that's all there is to it. However, the reality is that the situation is not that simple. If you were to do the above on your own, the executor that you selected would have to address the issue of probate, as the estate would have to go through probate or surrogate court proceedings. It is highly unlikely that your executor would know what this entails. For this reason, unless your executor happens to be an attorney, a probate lawyer is retained to represent the estate as it passes though probate.
During probate your creditors must be notified of your passing. Outstanding debts must be settled. Taxes must be paid. Property may need to be appraised and potentially liquidated. Dealing with fiduciaries may also be a part of the process your family experiences depending on the details of the estate.
Part of the probate process involves validating the authenticity of the will. If any interested party was to question the contents of the will, they would have to make those arguments in court, before the probate judge. Contested wills can also prolong the process of probate, which is already long enough for most people. The typical probate process takes between three and twelve months in most jurisdictions, but it can drag on for years in some extreme contested cases.
So if you were to create a will on your own without any knowledge of intricacies or probate court the probate lawyer who winds up handling your estate is going to have to deal with the will as it is.
On the other hand, if you consult with a probate attorney to help you plan your estate in the first place, it will be constructed with the probate in mind by someone with intimate knowledge of the probate process. There will be no questions about the legitimacy of the will. Your estate may well pass through probate quickly and efficiently. In a very real sense, when you are planning your estate you are in fact planning it for probate. Who could be better qualified to guide your family through the process than an experienced probate lawyer?