To save time. The Professional plan includes the full set of errors with suggested corrections, whereas the Lite version only shows a sample of the errors and suggestions to correct them. Moreover, the Pro version includes the edited version of your Word manuscript where each edit and suggestion can be identified by MS Word's Tracked Changes feature. You can go through and accept or reject each edit as you see fit. This takes hours off the time of going through each error one-by-one and making changes manually.