What is the benefit of using a fanout micro-service in nodeJS?

I was just reading this article HERE. Basically the user suggesting that a sample twitter architecture be split up into multiple micro-services , see the diagram below :-

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However, there is this one service though which i see multiple calls being passed though , not directly through user action but it sits in between. the Fanout Service, what exactly is a fanout service ? what purpose does it serve. So far what i understand is it sends data to multiple other services, But whats the advantage ? When exactly do we leverage a Fanout micro-service ? What value does it add ?