Is it legal for me to just write up my own Will? Why do I need this service?

What makes a document a legal Last Will and Testament?

To write a legal Last Will and Testament, you only need a blank piece of paper. At the top you would write the title, something like "Last Will and Testament" and underneath you would write something like "This is the Last will and Testament of me, John Doe, of 123 Main Street, Dallas, Texas". You would then go on to describe the distribution of your "estate", something like " when I die, everything I own should go to my wife, Jane Doe". You would then sign and date it at the bottom.
If you did this all in your own handwriting, in some States you would have prepared a perfectly legal Last Will and Testament. In other States though you would need to sign the document in the presence of two witnesses to make it legally binding. If the document is typed, then all States require it to be signed in the presence of two witnesses. No State requires the document to be notarized, signed by a lawyer, or registered anywhere. You will have created a perfectly legal document.
The problem is, it will be a really bad one!!!
Do not take this approach to preparing your Will unless you are pinned under a rock, or in some kind of desperate situation. 
A well drafted Will is usually about 5-6 pages and will start with a revocation clause clearly stating that any previous Wills are cancelled by this one.
You then need to appoint an Executor, the person who has responsibility to carry out the instructions in the Will. They also have to secure your assets, and gather everything up. This person has the authority from the courts to change locks on your house, and inventory your possessions so that nothing goes missing. They also have to arrange your funeral, pay of any debts, and of course, file your final year taxes. A good quality Will would also name a backup in case your first choice is unwilling or unable to serve.
A well written Will then goes on to describe the distribution of your estate. This may include charitable bequests, any specific gifts (for example, your favorite guitar to your nephew), and then a plan for everything else (your residual estate). The Will has to cover every scenario, like if you are involved in an accident at the same time as your main beneficiary, or another beneficiary dying before you.
If there are any minor beneficiaries, then a trust must be set up. If the minors are your children, then you can name guardians for them. You can also set up a lifetime interest trust for your partner if they are not the biological parent of your children. There are some quite sophisticated things you can do in a well written Will.
A properly drafted Will would also include powers being granted to your Executor; whether they can sell of your assets, whether they can place your assets into secure investments, whether they can hire professional help if they need it.
Writing a Will using a blank piece of paper is not advisable unless you are in an extremely desperate situation, but it is better than nothing. Writing a Will using a blank form, fill-in-the-blanks Will kit is not much better.
But using a service like the one at allows you to prepare a professionally drafted Last Will and Testament at about 95% less than the cost of using an attorney.