When to use it
Use it in any email where you're asking for something, especially to someone you don't email often. State your purpose in the first line. A pleasantry is fine, but if the ask is buried under it, people may miss it or put it off.
Example
You: “I'm writing to ask for your help with the onboarding guide. Could you review the security section this week?”
IT lead: “Sure, send it over.”
In an email or Slack message
In Slack: “Hi Sam, I'm putting together the onboarding guide and would love your help with the security section. Do you have 20 minutes this week?” One message, purpose first.