A good question.
We have all written our own contracts, whether we thought about it or not. When I was much younger, I gladly sold my ancient Toyota for $10 to someone who wanted to fix it up. The bill of sale was hand-written. Today, anyone can go online and draft a contract, will, power of attorney, or trademark application for a few hundred dollars or less.
When we negotiate any contract, we weigh the risks of making a mistake and the price of preventing one. We might pay our friendly mechanic $200 to inspect a used car before we buy it, but we probably won't hire a lawyer to draft the contract because most of us know what "AS IS" means!
What if the risk and complexity are greater? When we buy a home, we do well to hire an inspector for an expert opinion on its condition. Our lender hires a closing attorney to make sure that the house has clear title and that the seller swears everything is on the level.
What if you want to write a will or grant a power of attorney? In some cases, the risks might be low, but in most cases- NOT. I cannot recommend that you rely on an online form to direct the care of relatives who cannot care for themselves.
If you and your spouse were to die together in an accident, you might plan for such a tragedy in a carefully drafted will and make arrangements for the education of minor children or the care of an elderly parent. Sometimes the best caregiver is not good with money, so financial responsibilities can be separated from custodial arrangements.
Perhaps there are online forms to do these complex things, but are you sure they comply with your state's laws? If your online forms fail, do you want the probate judge to make these decisions about custody and money? There is a reason the legal form websites have lots of disclaimers. Does your family have at least one person who can cause trouble in probate court? Disability or death is not the time for a family feud.
The internet allows access to vast libraries of information at close to the speed of light. This is a good thing. But as T.S. Eliot said, "Information is not knowledge, and knowledge is not wisdom." Plugging information into online forms does not mean that the forms as executed will be wise to the delicate facts of your situation and the peculiar laws of your state. There are plenty of transactions which do not require the advice of an attorney, but when you are "playing for keeps" with your family's valuable property and estate, see a licensed attorney in your state.