Some people have asked me when they should use parentheses. The main use of parentheses is to show that the words enclosed in them aren’t essential to the meaning of the sentence. They provide additional information. Sometimes it might be information that doesn’t fit into the grammatical structure of the sentence. At other times they are used by writers to make a personal comment.
Here are two examples (taken from Oxford A-Z of Grammar & Punctuation):
He coined the term hypnotism (from the Greek hypnos, meaning ‘sleep’) and practised it frequently.
This is also known as junk email … or spam. Obviously it’s impossible to distribute processed luncheon meat electronically at this time (and hopefully it’ll never happen).
You can also use them to clarify numbers.
The new office furniture will cost about ten thousand dollars ($10,000).
Basically you use parentheses when something you write is a little bit out of place in your sentence. This is particularly used in informal English, perhaps in an email.
I’m heading out now (date night!!), but I’ll phone you to chat about the meeting tomorrow.
Hope this helps.