No! It is not necessary to use rewriting tool. It is designed for the people with poor English to fulfill their needs. If you are a professional content writer, then it’s also useful for you. It will enable you to increase your vocabulary and synonyms memory. Recently, we have added up to 500,000 synonyms in this tool for english language and it will enable you to create content with more professionally.
Never let your audience guess what you are trying to say, your writing should literally speak for itself. When your content has too many errors, it becomes difficult to read and understand. Although, one or two errors can be skipped by readers, but glaring grammatical errors can minimize the impact of your content. Also, it can make your writing difficult to read and digest.
Imo it is not possible for google to detect properly spun content, it is not worth computing power it requires. They can do this using ngrams for example, but it is uses a lot of queries. And for example article spinner I coded beats this algo, so google cant detect that the content is spun. Manual spinning is also good but it takes more than 30 minutes per article.