When pulling together a document from an online source or by yourself, there is no guarantee the documents used comply with local law. For example, it is easy to find yourself browsing through a contract made and enforced in Canada, Florida, or California. If you are not in Canada, Florida, or California, the same laws may not apply. Using these laws could mean the contract you spent so much time and effort trying to write yourself could be completely unenforceable.
The other issue is the documents pulled from the internet may not cover the situation you are actually involved with. For example, a sales contract may apply to the sale of goods, but not a car or trees on the property. Those areas of law can have special local issues, issues which may not be covered by the internet documents.
Think of an attorney like a car mechanic. Could you fix the car yourself? Of course, but most of the time it makes more sense to have the mechanic do the job. They know all of the electronic components, how to work with the engine, and where is the carburetor. This means they can fix the car in a matter of hours instead of a matter of days. Additionally, mechanics can look over a car that does not need immediate work and can provide preventative care so it costs less then if a part completely dies.
An attorney is the same. Attorney’s know what clauses for a will are and are not enforceable in any particular state. They know if a particular occupation is licensed and how that would effect the operation of a business. They can help navigate potential regulatory punishments and more. Attorney’s cost money and that amount can be significant. However, just like a mechanic, you can come and spend hundreds or thousands of dollars now to keep yourself covered. Or, you can wait until there is an issue and potentially pay tens of thousands of dollars to fix the problem.