There are two main reasons to batch resave:
Firstly it greatly speeds up the loading of patches once you have batch resaved them.
Secondly, it can help you find missing samples and relink them to the patches so that you don't need to search every time you load a patch.
Bear in mind that it can sometimes take a few attempts to batch resave, and if Kontakt crashes the first time you try, you could go into the instruments folder and batch resave a bit at a time -- go by sub folders for example, just to lessen the load on Kontakt.
Here is a youtube video showing the process.