It prevents unnecessary edits to have a preview button to test if the markdown is rendered as desired, before submitting/saving edits.
I searched for "preview" before creating this ticket with no results, so I think this may be the first formal request for this feature.
Just a note that I'd like to see this as well sometime in the future.
The preview feature is already available for comments on a ticket that has been created by myself, so this might be a just a matter of (wrong) permission checks. I guess this should pop up on every ticket i'm allowed to comment on.
But this feature would be really useful on ticket creating/editing.