You can buy software that produces most of the estate planning documents an attorney will prepare for you. Using such documents could turn out all right for you and your heirs, but things could go horribly wrong as well, and you'll never know if you did it right until it's too late. You could end up paying a nursing home unnecessarily or your heirs could pay unnecessary taxes or expend legal fees fighting each other.
Only a qualified attorney can educate clients on what issues they should be aware of in their individual circumstances and then recommend appropriate language to deal with the client's specific situation. Do you have a taxable estate? Do you own significant amounts of tax-deferred retirement plans? Do you know how to fund the revocable trust provided on the computer program? Is there anything about your estate that is unusual, such as having a disabled child? In short, if there's anything about your situation that's not plain vanilla, you need to see a lawyer. And only a lawyer can determine whether your situation qualifies as "plain vanilla." As with joint accounts, the problems you may create by not getting competent legal advice probably won't be yours, but may well be your children's. Do you want to risk leaving that legacy?