It is not necessary but oftentimes preferable. Assets that have beneficiaries listed on them are considered "nonprobate" and go outside the Will to those people you have named as beneficiaries. No probate proceeding is required to transfer the ownership of the accounts thereby saving time and money in processing those assets. I would suggest you review all of your beneficiary designations to make sure they are consistent and coordinated with your wishes as provided in your Will. There are also times where is is preferable to have all of your assets go through probate so that they are sure to pass in accordance with your Will. I would urge you to consult with your estate planning attorney to make sure your estate plan is comprehensive and coordinated.
The beneficiary designations are considered probate substitutes. Even with them, the will is a good back up if a beneficiary dies or you change accounts and forget to name a beneficiary.
That said, there are tax advantages to naming a beneficiary on IRA accounts. You should discuss that with your tax advisor.
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The will controls assets without beneficiaries.
The accounts with beneficiaries control themselves,
You should consider a trust to avoid probate on assets
Assets inside a trust avoid probate and guardianship plus other benefits-worth looking into.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
Necessary? No. If you have a Will in place, you can name "Estate of You" as the beneficiary and the funds will pour back into your estate. That is sometimes advisable. If you live someplace where there is an estate tax or you have enough of an estate that your heirs will be affected by an estate tax, there is usually a marital deduction or exemption. Routing money back into your estate can help fund a marital deduction trust and avoid significant taxes. There could be other reasons, too, such as planning of inter vivos trusts. Usually, for most people, if there isn't some reason not to, you should designate beneficiaries on policies, bank accounts, and IRAs. In most states, it doesn't save you probate costs. But, it does avoid probate and the occasional delays of probate. Planning an estate is complex, though, and it often involves specific circumstances. You can buy a cheap Will on the Internet, but you should see a lawyer to plan your estate.
Reading an answer on the Internet does not create an attorney-client relationship. You are represented by me when we have both signed a retainer agreement (on paper or electronically) and some money has changed hands. Usually, you will have been asked specific questions about your situation and all potential conflicts of interest will have been resolved. Until then, you have no more right to rely on this answer than if you read it in a novel.