It seems most developers I know have a story of running some sort of batch operation on a local Drupal site that triggers hundreds (or thousands!) of emails that are sent to the site's users, causing much frustration and ill will towards the site the developer is working on. One time, I accidentally re-sent over 9,000 private message emails during a test user migration because of an email being sent via a hook that was invoked during each message save. Filling a user's inbox is not a great way to make that user happy!
With Drupal, it's relatively easy to make sure emails are either rerouted or directed to temp files from local development environments (and any other environment where actual emails shouldn't be sent to end users). Drupal.org has a very thorough page, Managing Mail Handling for Development or Testing, which outlines many different ways you can handle email in non-production environments.
However, for most cases, I like to simply redirect all site emails to my own address, or route them to a figurative black hole.