Comparing data, of course. OK I'll expand on that. I'd say the most common thing I have to do regularly is prepare website updates. I make a habit of creating development & production environments that are practically identical to eachother. And whenever it comes time to make some database updates I'll do so in development first so I can test everything out. My goal is to iron out any bugs in the development environment first, and then copy over everything I changed bit for bit.
Now, one could do this with an import or export through Microsoft SQL. But in some cases this can be a destructive process, essentially taking your website down and throwing errors as the transfer takes place. And that's generally a bad idea if you're concerned about your users.
OK, the other option is to manually and tediously look for the changes you made and make them one-by-one in the production database. The problem here is that you'll often overlook something.
So the happy medium is Data Comparisons. Using it to correct differences is still a manual process, but with Data Comparisons you can find those differences faster. And at the end of the day you can be confident that the tables you're working with are indeed identical.