Why do we use meshes such as "Iso mesh" , "Paver" and Hybrid" mesh in structural analysis?

http://www.mscsoftware.com/training_videos/patran/Reverb_help/index.html#page/Finite%20Element%20Modeling/mesh_forms.03.2.html

Please follow this link to understand the concept. It is specific to MSC softwares, so you have to get into the MSC documentation for details especially PATRAN.