Hi headline
Great question.
The main purpose of use is when you require irregular data sets to be stored and shared between plugins. An example maybe storing times for a timer plugin. You would not be required to store arrays for level / player times and could simply use a dynamic object for storage.
You could also hook member data changes which will call forwards to each plugin that has hooked the dynamic object. With this your plugins can be aware of changes in real time without constantly checking for updated data via natives or global forwards.
You wouldn't use dynamic objects for plain arrays that dont need to be shared via other plugins.
If you've ever had a moment where you wish you could make your own netprop. This is a viable solution that would allow such functionality without the need for you to write natives.
I'm personally using this to create member sets for each player. Where one plugin can store members like a playerid and others can read them easily and or hook member changes. I also hook changes and use them to update a mysql databases to synchronise changes automatically to and from plugins with no effort at all.
I have some planned changes where <dynamic>(1-maxplayers) will always relate to data for the corresponding client index that auto initialises and disposes with client connect and disconnect events.