It all means that opposite forces have to exist. We have to contend with both. If there were no enemy, internally, to struggle against, life could be totally without any challenges and therefore uninteresting. If everything was going smoothly, then we would not even recognize ‘good’ as ‘good’.
The cross-currents of conflicting powers exist for the purpose of proving to us that ‘good’ will survive and surface at the end. The ‘desire’ in Brahman resulted in the creation of the universe. The ‘desire’ in the living results in the world of the living growing up.