Update[symbol] tells Mathematica that hidden changes have been made which could affect values associated with a symbol.
Update[ ] specifies that the value of any symbol could be affected.
Update manipulates internal optimization features of Mathematica. It should not need to be called except under special circumstances that rarely occur in practice.
One special circumstance is that changes in the value of one symbol can affect the value of another symbol by changing the outcome of Condition tests. In such cases, you may need to use Update on the symbol you think may be affected.
Using Update will never give you incorrect results, although it will slow down the operation of the system.