Good question. This is a deviation that Bhante Vimalaramsi did. He took it from the Visuddhimagga. It turned out to make the meditation easier for people to start with. It’s like using training wheels for a while.
Instead of sending Metta to all directions and all beings as per the suttas, you first start with just one person that you admire - someone who makes you smile, when you think of them.
You start small, as your feeling of Metta is still weak and is just getting started. It’s much easier to dwell on one person, than on all beings at once.  The Metta initially gets too diffused or watered down when it goes into the directions.
So, you start with a Spiritual Friend to get familiar with the process of the 6R’s. Once the Metta has built up to the level of the fourth Jhana, you will learn to radiate in the directions and to all beings.
Contact a teacher here if you’re not sure whether your practice has progressed to the fourth Jhana. They will advice you on how to progress.
Please check below if you want to know more about how to pick a suitable spiritual friend:
https://www.dhammasukha.org/getting-started-with-twim.html