Why Do We Have Two Hemispheres?
As logical processing and database management have different execution strategies, they cannot be executed in optimized manner using common processing resources. For the same reason, the optimizing aspect of the evolutionary process has divided brain into two hemispheres, where, in a majority of population, logical processing happens in the left hemisphere and database storage and management happens in the right hemisphere (along with limited logical processing capability).
The human brain is divided by the evolutionary process to execute logical processing and database processing separately.
Based on such distribution, all creative, emotional, imaginative, rational, thoughtful, holistic, artistic, introspective, etc. interactions (or parts of such interactions) are processed in the right hemisphere (besides other functions), as they actively require past data from the database (i.e. information stored from past experiences), whereas logical, analytical, objective, detail oriented, rule based, factual, repetitive, critical, orderly, systematic, etc. interactions (or parts of such interactions) are processed in the left hemisphere (besides other functions), as they do not actively require past data. Such distribution of brain functions between the hemispheres is called "lateralization of brain functions".
(Above is the first ever causal explanation of why the human brain has two hemispheres and the reason brain operations are lateralized)
Detecting Lateralization: Based on the above, one can argue that if the brain works in such clearly dichotomous manner, the same should be easy to detect by observing their activity using fMRI or other brain imaging techniques, which is not the case, the reason for which is that processing of a majority of interactions is distributed dynamically at intra-process levels in varying degrees between both hemispheres (as explained in the next topic), not considering which makes it difficult to detect such dichotomy via observation.