For instance, quality assurance specialists need to find changes and analyze how these changes will affect other documents. Manager might need to check someone's job and provide report about project progress. Web developers are interested in comparing web page files to highlight changes that were made.

Some programs has a built-in compare function. For instance, Microsoft Word has an ability to edit documents and more over, do document audit. It's possible in Word to display what changes were made and who did these changes. But there are some limitations in Word, for instance, sometime the report is too detailed and it's hard to find out what changes are really important.

For sure, a number of well-known compare tools suggest a compare function to end user. These tools can handle most file types, but sometime lack an ability to represent source file format correctly. For instance, comparing Word or PDF file will be hard as most tools doesn't represent actual document formatting. More over, it's not possible to specify what formatting is important to you right now, e.g. some font changes may be very critical to you, but color changes don't mean much.

As for comparison built in Word, it's very convenient, as you don't need to have any third-party utility. What you need is just a Word and two files to compare. Give a try to this feature before considering purchase some third-party tool.

What do you miss when you use just Word comparison? Well, it works good for documents that were changed just a little, but when you need to compare documents that were changed a lot, then Word will not do it's job. In this case you might need to use some tool that supports comparison by keywords.

Also, if you wish to share the results of your comparison with others, then it would be hard to get some kind of comparison report, put your comments there and process it to your colleague.

Finally, with Word comparison you will not be able to compare Word document against Excel or Word document against PDF file. Word will deal only with documents of Microsoft Word software. That is the common limitation why people are looking for some other compare tools.

MS Word software is great for comparison of Word files, but it will hardly compare documents that were changed a lot or documents of different type. That's why, if you are involved in quality assurance or need to control document changes, it's a good idea to pay attention to third-party compare tools.