These two options in the Review tab are often unnoticed. Here is what they are for.
If you have an original document and another revised (changed) version – but you did not enable Track Changes before changing it – you need the COMPARE option. It will take both the documents, compare them and create a third document showing Tracked Changes.
If you have sent a document with Track Changes ON to many people and all of them send their revised version to you. Now you have multiple documents with each containing tracking by a DIFFERENT person. In this case you use the COMBINE option – two documents at a time to MERGE the track changes. Of course, changes are not accepted – just accumulated. Try it!