Decreases the chance of your emails being marked as spam by adding a plain text version to your HTML emails.
Because it decreases the chance of your legit email being marked as being spam and thus landing (disappearing) in spam folders.
No, the plugin does not affect the HTML message of emails.
No, 99% of all email clients will just show the HTML version of the email.
The other 1% can't properly display HTML and will show the plain text email. Without this plugin, they would have had nothing to show.
You can use the following online tools to test the spammyness of your WordPress emails.