This is legacy code from older examples that isn’t needed with Mongoose 5.
Mongoose 4 relied on its own promise implementation, mpromise. mongoose.Promise wasn’t necessarily Promise global.
As Mongoose 4 documentation states:
Mongoose 5.0 will use native promises by default (or bluebird,
if native promises are not present) but still
support plugging in your own ES6-compatible promises library. Mongoose 5.0
will not support mpromise.
Though the statement about Bluebird is no longer true; Mongoose 5 dropped the support of Node versions that don’t have native promises.
mongoose.Promise = global.Promise;
may still be needed if global.Promise was assigned with another implementation (e.g. Bluebird) after Mongoose was imported, though a better thing would be to assign global.Promise = Bluebird earlier instead.