The pardot classic way of email building doesn’t provide you with a mobile view in the build phase of the email. Your options were HTML or text.
But most people (well…all?) look at emails on their phone first, laptop second. The mobile interface is the primary way an email is viewed so it makes sense to be able to see how your email will look as you’re building it.
For example, you might think your text looks better aligned to the left on desktop, centred on a mobile. You can edit both in the interface and not affect the other.
With the email builder, you can switch to a mobile view and alter the layout for a mobile screen that won’t affect the layout of the desktop view…two layouts depending on the screen being viewed.