Microsoft Is Deprecating The Legacy Web Interface
The traditional way that you access Dynamics 365 CE is through the classic interface on the web. Microsoft now has a new method using the unified interface. The unified interface means that your forms will work equally as well on tablets and phones.
Another important thing to note is that they are discontinuing the classic/legacy interface very soon. Microsoft has announced they do not plan on investing in the legacy interface anymore, as their focus shifts towards the new unified interface.
Why Should You Transition to The Unified Interface?
Microsoft published these stats that put the unified interface at a higher pedestal than the legacy.
63% improvement in grid load
37% improvement in form load
45% improvement in dashboard load
30% improvement in quick create load
Here is are some links to articles where Microsoft explains why and when they are discontinuing it: