Apps are designed to enable usage without requiring detailed understanding of programming and so they can be applied in batch to a large amount of data. In other words, being able to use them as functions without writing loops or conditionals.
Apps can be used by themselves, or added together to define a “composed function” (aka pipeline). Composed functions can be applied to a single, or thousands, of data file(s).
Apps have several key features
check the validity of input data
can be used by themselves or combined into a “composed” app
automatically track the relationship between an input data record and its output record