What's the purpose of "if not then require" pattern?

[ugultopu]

I have observed the following pattern being used in many places in the codebase:

let varToReferAModule;
// Other code
// Inside of a function
if(!varToReferAModule) varToReferAModule = require('path/to/that/module');

What's the benefit of using this pattern instead of simply using:

let varToReferAModule = require('path/to/that/module');

wherever varToReferAModule is declared?

Examples

Stream Promises:

let eos;
// Other code
if (!eos) eos = require('internal/streams/end-of-stream');

---

eos, EE, PassThrough and Readable in Internal Streams Pipeline.

[devsnek]

We use it to lazily load functionality, so that Node.js can start faster.