Why are parentheses used in a sentence?

A parenthesis is used in a sentence to indicate that you wish to interrupt your own sentence to include some comment or information that is incidental to the actual point of statement that your sentence is making. This is actually quite similar to the use of commas, which are used to separate different clauses of a sentence so that they don't all run together, but the parenthesis does so with greater emphasis, it sets something apart more forcefully than commas do.