What is a Transaction?
•A transaction has four properties:  It is “Atomic, Consistent, Isolated, & Durable”. 
•Atomic - A unit of work that succeeds or fails.
•Consistent - System restored to a constant state after completion. •Isolated - one transaction does not impact another, even if done at the same time. •Durable - A committed transaction is permanent.
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In order to understand why CICS is necessary, you must understand what a transaction is.  The definition presented in the article by Comaford is quite common and is known as is ACID definition.  ACID stands for Atomic, Consistent, Isolated, Durable. [Comaford]
Atomic means that a transaction will operate on one or more tables and rows such that either all of the changes occur or none of the changes occur.  This group  is known as a unit of work (UOW). A Commit occurs when all of the changes succeed and the changes are made permanent.  A Rollback occurs to reverse the changes and the database is restored to how it was before the start of the UOW.
Consistency means that the database conforms to all of its rules at the end of any work.  For example, this might mean that no two row identifiers are the same.
Isolated means that changes performed by one person cannot be seen or interfere with anyone else until they are committed. It is as though you have your own private database.
Durable means that once changes to the database are committed, the change can live through a system failure.  Uncommitted changes cannot survive a system failure. [Nusphere]