According to Magento2 Doc By default, when you install a package, Composer puts it into vendor/<package> directory, such as vendor/magento-module-catalog. However, the Magento framework requires Magento components to be in different locations, such as app/code/Magento/Catalog. The process of copying or moving contents of packages from default to the necessary location is called marshalling.

What is the benefit of copying it into vendor/<package> first and then moving it into something like app/code/Magento/Catalog ?