Capability: Typically you will go through many iterations of your code and the work products your code produces. It quickly becomes impossible to track changes and reproduce earlier work without some code version control tool. This is only exacerbated when your team size is >1.
Capability: Even if your data is small enough to fit in memory, reproducing work will involve running all those scripts into memory before you can pick up where you left off. Other team members have to do the same. This is painful and inefficient. You therefore need to persist your work (raw data, intermediate datasets and work products).