Why email yourself a reminder instead of using a calendar alert? Each has its place; here's where email reminders shine.
No app to open. Everyone already checks email. A reminder in your inbox meets you where you already are, on any device, with nothing to install.
Nothing to set up. Calendar apps often want an account and configuration. An email reminder takes one screen and no sign-up.
It waits in your inbox. A calendar notification flashes and disappears. An email waits patiently as an item you can act on, forward, or archive.
You can still have both. Prefer a calendar entry too? Tick the calendar option and your reminder email carries a .ics file you can add to any calendar.
Email reminders aren't a replacement for a full task system — they're the fastest way to make sure a single thing comes back to you at the right time.