This is not intended as legal advice, and only covers general knowledge I have on the subject of wills as a result of handling my own finances.
Each state of the USA has its own laws on wills and trusts. You can find these online. For example, in Kentucky I found state laws here: http://www.lrc.ky.gov/krs/titles.htm and Title XXXIV is about wills and trusts.
I would recommend reading this, and then talking to a lawyer if it is not crystal clear.
Generally, if a lawyer does not draft your will, then either (1) you have no will, or (2) you use a form or computer program to make a will, that must then be properly witnessed before it is valid. If you don't have it witnessed properly, then you have no will.
In some states you can have a holographic will, which means a will in your own handwriting. That's when you have that 3am heart attack, and you get out a pad of paper and write "I rescind all former wills hereby bequeathing everything to my mistress Samantha, and as to the rest of you go rot in hell. "
One issue with these is that they have to get to court somehow, and someone has to verify the handwriting, and there are often state laws about excluding a current spouse, so you can guess for yourself whether that one might disappear in the fireplace when another family member finds it next to the body or if a court would give it validity. And there can be logic or grammar problems with do it yourself wills, made in your own handwriting, without experience or good references on how to write things out. Lawyers who have done a bunch of these know what is clear and makes sense.
(1) In Tennessee, where I live, an intestate's property, someone who died with no will, is divided according to the law. The law looks to find a spouse or relatives to divide the property, before considering giving it to the state. That might be fine for some people. It happened once in my family, and was resolved in court with minimal red tape. But it really depends on the person. Someone in the middle of an unfinalized divorce, for instance, probably needs a will help to sort out who gets what.
(2) A form will is valid in Tennessee if it is witnessed properly. That means two witnesses, who sign in yours' and each others' presence. In theory they can be called to testify that the signature is valid. In practice, I don't know if this happens as I am not a lawyer. I have found it difficult to find witnesses who will sign a form will, and it is disconcerting to have to ask friends or coworkers for this sort of favor as most people learn never to sign anything without reading it. But a lawyer often has secretaries that do it...
There is a procedure and a treaty for international wills, which I know about from living overseas. To streamline things, you can get the witnesses to each sign an affidavit after they signed the will. The affidavit is sworn written testimony of what happened, that they saw the person sign their will and sign in each others' presence, when, where, no duress, etc. If done correctly, this can be sufficient to prove the will without calling on witnesses.
There is another option (3) you arrange your affairs so that most of your funds are disbursed by banks or brokers holding your accounts. Option (3) is really cheap, most stock brokers and banks will create a Transfer-On-Death notice on your account for free. The problem with this is that you also need to write out a letter that explains to your heirs how to get this money, and you need to make sure that they will get the letter if you are dead. Also, you can't deal with physical goods or appoint a guardian for children this way.
The advantage of a lawyer is that you know the document is correct and according to local law and custom, and also the lawyer might provide additional services like storing the will in his safe. You can get personalized help that you can not get with a form or computer program.