While many research students go for professional editing help, it is not substantive editing that is always required for your dissertation. If you have native-like command on the English language, then you may not be the one needing substantive editing support. However, it is most required when a student is unaware of basic linguistic rules and academic writing standards. Substantive editing should be availed when you are not confident of your command on the language or you are an ESL (English as a second language) student.
In the above cases, your content and language may not do justice to your hard-done research work. Thus, it will highly require making your work meaningful and logical through substantive editing. This type of editing or review can correct all basic and advanced linguistic errors in your dissertation. Additionally, substantive editing done by a professional editor can make your content more presentable, clear and concise. It can even do fact checking and correct your referencing styles.
If you do not have knowledge about professional styleguides and the writing styles to be adopted in your research area, then it is again time to go for professional help. An editor can assist you in complying with the rules and standards of the style manual recommended by your university. Through substantive editing assistance, you can make your work flawless, as well as can add value to your content. If you do not own a rich vocabulary, then do not worry. Substantive editing improves upon your word usage and paragraph logic too.
Thus, the absence of effective command on the language will not be a barrier for you when you take substantive editing help. In fact, you should go for professional support as soon as you are ready with a rough draft of your dissertation. A professional editor can take care of the rest.