If you will be the maintainer of a self hosted redash service – you must be proficient with Linux , this will help you to get through Chapter 2, Installing Redash chapter.
If you will be using Redash to write and visualize queries only – then your minimal requirement will be SQL (no matter which).
If you wish to contribute back to redash, or extend its functionality – then Python and some JavaScript knowledge is required.
In all of the above cases – you can only benefit if you run the examples presented in the book.
from dev side – most of the benefits come from looking at the code!