On mobile, visibly displaying key areas of an app's information structure in the top level nav menu is usually better than making the user first click a hamburger or other icon. There's a good illustration of this principle in the difference between the native New York Times app on iOS and Android.
On iOS, a small arrow in the upper left is how you expose key information categories. If it's your first time in the app, it may take a moment to figure this out.
The iOS NYT app requires a tap on the arrow in the upper left corner to expose key information categories.
The experience on Android is noticeably better. Categories are out in the open on top in a nice scrollable ribbon and hard to miss. There is no question where you are, and it's easy to see other options.
When it comes to reading email on mobile (the world's dominant platform), we're all like over-amped weekend skiers schussing the blue runs. All we want to do is go straight down as fast as possible. Over and over. What we do NOT want to do is have to decelerate and make a bunch of awkward wide turns all over the slope. That's no fun at all.
But that's what email that has not been formatted correctly for mobile forces us to do.
In our everyday digital travels, we grow accustomed to certain patterns and interactions. Best practices evolve quickly. As soon as there's a better way to do something, it becomes the norm. The UX bar is only raised over time - never lowered.
With millions and BILLIONS of consumers now expecting great design and user experience, getting even a small detail wrong can be a problem.
Here's an example of how a seemingly tiny design issue actually makes a noticeable difference in everyday usability.