One of the most common reasons is to continue to self trustee the plan. There may be a number of assets that are not handled by most IRA custodians that the trustee is perfectly capable of handling. In some cases if there are notes, mortgages, rental properties, etc. creating monthly transactions, having a self-trusteed 0% MPP can cost less than IRA custodial fees since the trustee can handle the receipts, payments and banking personally.