After all, the adjective wonderful is formed from the noun wonder. Still, English speakers understand intuitively that wonderfulness is the state or condition of being wonderful. It makes sense even if it is not great English. And in some cases the -ness suffix can add a subtle difference. Faith and faithfulness are both good English words, but they have a different meaning today.
Faith is trust, while faithfulness is the ability or quality of maintaining or keeping trust.
There are many other words which the -ness suffix had in fact made a new word with a slightly different meaning. This is not usually a problem for English writers except when the root ends with the letter n . For example, the words brokenness, barrenness, or drunkenness all need to have the double n because the root ends with an n and the suffix begins with another n. It means the wife of or female heir to a baron. A spell checker can help you a lot, but if you are still not sure, give Grammar Slammer Deluxe a try.