Plaintext versions of HTML email
Setting a plaintext version of your email ensures that email clients can correctly display previews of your email message.
In the absence of a plaintext version, Gmail will try to generate a version of its own. This quite often leads to mangled or confusing text - not what you want in that valuable but small space to get your point across.
We recommend as best-practise to always hand-craft your own plaintext fallback message, so it strikes the right tone with your recipient.