[commandlinefan]

> to ensure the code works correctly

It's to ensure the code works correctly and indicate where the problem is, whenever it doesn't work correctly. Querying a live database during unit tests fails on both accounts. It doesn't tell you whether or not the code works correctly - it tells you either that the code didn't work correctly or that the database wasn't available at the time the test ran.

[goto11]

Well both things are problems which is nice to know about so you can fix them. Certainly better than not knowing about either problem.