Why is it important for marketers to optimize their mobile experience?

You want to help consumers find what they want to find, and do what they want to do, quickly and easily.
Trick question. Mobile optimization was popular in 2015, but due to changes in the Google algorithm, it’s no longer necessary.
It’s cheaper to buy website themes with responsive design templates included, so you might as well optimize for mobile.
Device type directly implies location or intent; optimizing and capturing leads on mobile can help you create a better segmentation and lead nurturing strategy.

The correct answer is:

You want to help consumers find what they want to find, and do what they want to do, quickly and easily.