Estate planning is unique in the sense that it intersects with so many other areas of the law. It's not uncommon that a client's estate plan might raise issues related to real estate, business, employment, insurance, intellectual property, and taxes. On top of that, a skilled estate planning lawyer will generally have excellent interpersonal skills and navigate delicate family or other issues with common sense, objectivity, all while maintaining the highest levels of confidentiality and ethics.
One of the primary reasons that I recommend hiring a trained estate planning lawyer rather than taking a DIY approach is simply because of the experience the lawyer has. He or she has probably seen the situation you're facing hundreds, if not, thousands of times. The lawyer will usually have helpful suggestions on how you can transfer your assets in the most efficient manner (whether from a tax perspective or otherwise) and can often serve as the intermediary for bringing comfort to the family about a particular decision you've made (for example, the selection of one of your children as the manager of a business over the others after you've passed away).
Estate planning lawyers can also serve as a sounding board for potential business and personal situations that come up in your life. Lawyers are trained to view facts objectively, and can play "devil's advocate" to expose you to a different way of viewing your issue. In the backdrop, the estate planning lawyer will be assessing your situation in light of laws that may be implicated as a result of your decisions.
Certainly utilizing a document preparation service can save you a few dollars in the short run, but over the long term, hiring a competent estate planning lawyer could save you quite a bit more.