The answer depends on which English translation you are using. The original Hebrew (Old Testament) and Greek (New Testament) texts have no parentheses, periods, commas, quotation or interrogation marks etc.
The parentheses in the KJV are simply to aid the reader to make sense of the English text. It does not mean that the words in parentheses are not found in the original. In the KJV words added by the translators (again to make sense in English) are printed in italics. This can be seen in Matthew 1:6 where ‘that had been the wife’ does not appear in the Greek but is used to make sense in English.
Below are a couple of examples of words in parentheses.
Matthew 6:31 Therefore take no thought, saying, What shall we eat? or, What shall we drink? or, Wherewithal shall we be clothed? 32 (For after all these things do the Gentiles seek:) for your heavenly Father knoweth that ye have need of all these things.
Matthew 24:15 When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:)
In these two examples it is not difficult to see why the translators used parentheses.
Modern versions sometimes follow the KJV in the use of parentheses and sometimes use them in different places.
I have read, but I am unable to verify, that there are 223 verses with parentheses in the KJV, 269 in the ESV, 189 in the LITV, 240 in Darby’s Translation, and 240 in the NASB.
The Amplified Version adds explanatory expansion to the text within parentheses to separate the additions from the translated text.
Some modern versions (for example the ESV) also use parentheses to indicate that the words do not appear in the manuscripts preferred by the translators. Others use footnotes.
In conclusion: parentheses used by the translators are intended to aid the reader. In most cases the passage can still be easily understood with the parentheses removed.
I hope you have found this helpful.