What is the use of posting messages and then deleting them right away?

I see several posting with interesting subject headings, but when I click on them, I find that red X in a circle which means (I presume) that the poster deleted them.

What is going on here? If these types of posting are quick in-and-outers, why not just send an email to the one you intend it for?

I seek not controversy but enlightenment.