This one is important as Google is led by a mobile-first index nowadays.
For eCommerce and other stores, a mobile app makes sense, especially for an established brand. You can easily log-in with your social accounts or Google. Additional features using your GPS may pick the nearest store for you, and paying with something like Google Pay or Samsung Pay is even easier.
From a design standpoint, you will need a different experience than desktop. Due to the limited screen space, picking the right elements that a mobile user will find intuitive matters. This will increase your conversion rate or whatever you are after.
This affects the overall user experience. Submitting a long answer on mobile is not intuitive. Filling out a long form or reading long paragraphs, too. Consider the limitations of mobile and act accordingly.
This last lesson is valid for all sorts of marketing. An email lead capture form should be small – unless you incorporate Facebook/LinkedIn login which would pre-fill these for you.
Social media – consider how your ads will look within the mobile apps.
Follow this reverse-engineering strategy and you’ll get a head start.