A rich stdlib establishes patterns and provides standard types for other libraries and frameworks, making the whole thing generally more consistent as opposed to something like the Node.js ecosystem, which looks and feels more like a bunch of random parts crudely fitted together through copious use of duck tape and glue.
Concretely, Go has eg context.Context, net.Conn, http.Handler and of course io.Writer/Reader/Closer. All 3p libs will use the standard types. This means you can compose many 3p building blocks together, without any prior knowledge or coordination on their end.
When you have an insufficient stdlib, you often get compatibility issues, where things don’t compose. So you get these kind of pseudo-std mega frameworks like Tokio instead. It was a while ago I was deep in JS but I remember similar things there.