When to use it
Use it when someone asks for information you've already sent. Resharing it with a neutral line saves them a search and keeps things friendly. “Per my last email” is widely read as annoyed, even when you don't mean it that way.
Example
Colleague: “When is the deadline again?”
You: “Resharing the details below so they're easy to find: deadline is March 3, and the brief is attached.”
In an email or Slack message
In Slack, link to the original message: “Here's the earlier thread with the details: [link]. Short version: March 3.” Adding the short version saves a click.