Do you ever need to calculate a field based on the contents of another field?  Sure you do. Here's the easy way to get 'er done, with the added flexibility of conditional output based on the input data.

This toy example pulls data from a numeric field and feeds the attribute value to a function that tests the value and returns a text description to a different field. 

It's not a big deal to select by attributes and then do a field calc on the selection set, then repeat for the next selection, say numbers > 1000, it's just more fun to do it with code since it opens up more opportunity. What if you wanted to create a directory on your network based on the calculations result?  Well you could do that with python by tucking a little extra code in your condition.