You are tasked with building a UI - and it must work in a pc-based browser and a mobile-based browser. For the mobile-based browser, you have to accomodate both portrait and landscape orientations - in the same session. This use case represents a lot of variables to try and account for, and it is unlikely you will work out a solution for all of these variables.
With dynamic UIs, you only need to focus your development effort on the "content" and Pega 7 will take care of the "presentation." Using a dynamic (responsive UI) layout means the UI will automatically adjust to the user's device. They might see three columns in a row when using a pc, two columns in a row when using a tablet in a landscape orientation, and only one column when using a smart phone.
The "advantages" of using dynamic UIs are not unique to Pega. Do a web search for "responsive UI" and you will see that this is very standard practice.
The "advantages" of using Pega 7 is the very shortened development effort it takes to produce these repsonsive UIs. When building UIs, use the dynamic layout. Then, all you need to do is specify a few parameters and Pega 7 will take care of the rest.