Purity is the main concept behind all of the former mentions. Purity helps to find weaknesses in design for data insecurities and helps on the other side to stabilize systems due to its cleaness. There is nothing unneeded which might get broken. There are only parts used which are really needed. One can concentrate on these to get them stable and secure.
Purity also helps to increase speed due to its leak of unnecessary work to do and to get the software usable by clear, open and easy graphical user interfaces or programming interfaces.