Let's say you have a page called "Page A". You decide you want to put a 301 redirect in place for "Page A".
When you do this in Siteglide, the system deletes all the HTML content that was in the page you're redirecting.
Here's a 1min video of it happening:
This is problematic because often times we are redirecting a page because it's out of date, but we need to reference it, to make a copy, or take some HTML from it for a newer version we're redirecting to.