Do I need to use an asset packager with Ruby on Rails?

I am researching asset packager gems for Rails. I found out that Rails has its native solution to this problem in the cache=>"all" option on the include tag helpers. There is also some discussion about whether this is good enough, and some gems like Jammit have their diehard defenders. If there is a native solution to packaging javascript assets, why do we need an asset packager gem?