Greg gave an excellent answer. The Oxford Learner's Dictionaries website is an excellent resource for English learners. https://www.oxfordlearnersdictionaries.com/definition/english Here is its explanation and examples for the pronoun "one." https://www.oxfordlearnersdictionaries.com/definition/english/one_2 one pronoun BrE /wʌn/ ; NAmE /wʌn/ Explanation: ​used to avoid repeating a noun, when you are referring to somebody/something that has already been mentioned, or that the person you are speaking to knows about Examples: I'd like an ice cream. Are you having one, too? Our car's always breaking down. But we're getting a new one soon.

'One' is a pronoun that refers to what is being talked about - car in this instance. Using 'one' saves you from having to repeat 'car' in the sentence.

1 The car wasn’t good. (Fine) 2 The car wasn’t a good car. (It’s redundant to repeat car. It’s not wrong, though, and there could be times where you would do that on purpose) 3 The car wasn’t a good one. (To avoid repeating car.More natural) 3 and 2 sound a little more definitive than 1. You’ve thought about the car and determined that it belongs to the category of “good car”.

It’s not to repeat the noun. If you said “the car wasn’t a good car” it would sound a bit redundant, that’s why you’d replace the second noun with “one”.