What does "set(node_plugins ...)" actually do?

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I notice that composable component-based makefiles like this one have lines like:

set(node_plugins "${node_plugins}composition::Talker;$<TARGET_FILE:talker_component>\n")

Where exactly is this being used? It just seems to essentially duplicate the previous line:

rclcpp_components_register_nodes(talker_component "composition::Talker")

But with the added fun of a hard-coded variable name. I've omitted the line and things still seem to work for me.