How does request.context in Parse Cloud Code work?

Back in Parse Server 3.0 update, there was an addition of request.context to pass data between BeforeSave and AfterSave as documented here:

https://docs.parseplatform.org/cloudcode/guide/#using-request-context

However, I'm having a bit of trouble understanding how and when Parse runs this code in the example.

const beforeSave = function beforeSave(request) {
  const { object: role } = request;
  // Get users that will be added to the users relation.
  const usersOp = role.op('users');
  if (usersOp && usersOp.relationsToAdd.length > 0) {
    // add the users being added to the request context
    request.context = { buyers: usersOp.relationsToAdd };
  }
};

const afterSave = function afterSave(request) {
  const { object: role, context } = request;
  if (context && context.buyers) {
    const purchasedItem = getItemFromRole(role);
    const promises = context.buyers.map(emailBuyer.bind(null, purchasedItem));
    item.increment('orderCount', context.buyers.length);
    promises.push(item.save(null, { useMasterKey: true }));
    Promise.all(promises).catch(request.log.error.bind(request.log));
  }
};

in other examples, cloud code functions are run via Parse.Cloud.beforeSave or Parse.Cloud.afterSave. In this example above, the function beforeSave is assigned to a

const beforeSave.

Why was this done and is this supposed to be placed inside main.js top level or inside another function?