When discussing the use of the WiX toolset sometimes it is helpful to be specific about where the product is in its lifecycle. We break the product lifecycle into three general categories:
A product begins its life in "build time". This is where the product is created. The WiX toolset usually plays a significant role during the latter phases of "build time" when the product is packaged, signed and released.
The bulk of the WiX toolset is used during build time.
When a product is deployed it enters "install time". This is where the customer's first impression of the product is formed. The WiX toolset functionality used in this category is generally written in native code to keep the dependencies to a minimum.
Only select portions of the WiX toolset are used at install time including: the Burn bootstrapper engine for Bundles and custom action DLLs from Extensions.
Hopefully, a product spends the bulk of its life doing what it was designed to do in "run time". Very little of the WiX toolset participates in the run time of a product although some components, such as DTF, may be useful.
The cycle repeats for updates to the product.