BSP and mesh Objects UE4

Hi there Everyone,
Im very new to Unreal engine 4, and have debled a bit here and there with the level design tutorial on the site.

I would like to understand how the whole BSP and Mesh objects are saposed to work for level design.

This is my thinking.
You create a level with the BSP Brushes. Once you are complete, you can then go into a 3rd party app, Maya, Blender etc and then create the pieces needed to make up your Level.
Now thats fine, and i wont have a problem with that. But id like to know what that “Convert to Static mesh” option is all about.
Dose it create Clean geomatry for optomised play or is it still better to create your own ?
Any advice would be great, and what are your work flows ?

Thanks so much for your help and advice.