“Punctuation, then, is the use of spacing, conventional signs and certain typographical devices to promote understanding and to guide correct reading, whether silent or aloud.”
— John McDermott —

PARENTHESES

Either or both of a pair of signs used in writing to mark off an interjected explanatory or qualifying remark.

Use parentheses ( ) to include material that you want to de-emphasize or that wouldn't normally fit into the flow of your text but you want to include nonetheless.

If the material within parentheses appears within a sentence, do not use a capital letter or period to punctuate that material, even if the material is itself a complete sentence. (A question mark or exclamation mark, however, might be appropriate and necessary.) If the material within your parentheses is written as a separate sentence (not included within another sentence), punctuate it as if it were a separate sentence.
Forty-three years after his death, Robert Frost (we remember him at Kennedy’s inauguration) remains America’s favorite poet.
Forty-three years after his death, Robert Frost (do you remember him?) remains America’s favorite poet.
Forty-three years after his death, Robert Frost remains America’s favorite poet. (We remember him at Kennedy’s inauguration.)

If the material is important enough, use some other means of including it within your text — even if it means writing another sentence. Note that parentheses tend to de-emphasize text whereas dashes tend to make material seem even more important.

— Capital Community College Foundation
Guide to Grammar and Writing —

Place a period outside a closing parenthesis if the material inside is not a sentence (such as this fragment).

(An independent parenthetical sentence such as this one takes a period before the closing parenthesis.)

— The Associated Press Stylebook —