Let’s take that second question first. You need them because the IRS says so! The Custodian and Administrator are often one in the same, but can be different. The IRS requires that you have a custodian for your IRA, regardless of whether you use the IRA to invest in alternative or publicly-traded assets. The custodian holds title to your assets for your benefit and is responsible for IRS reporting, while the Administrator executes your investments according to your instructions. For more about the role of a custodian, this publication from the Retirement Industry Trust Administration is helpful.
You can find a full list of Transferring Custodians, average transfer times, and contact information here.