When testing software or systems implementation, you want a database into which you can record your 'bugs' or defects. This database can also sometimes be used as a requirements gathering tool and/or a customer service tool.
Without such a thing, you will be using email and excel - no unique numbering, date stamping, no visiblity into status or assigning, no version control... I could go on and on.
There is also GNATs but it requires a bit of computer literacy on the Unix side.
Bug databases have their uses too when analyzing what your most common defects are. You can then change the standards or design procedures to avoid those defects.
Another slant is what customers complain about. It may be the most trivial thing like they have colour blind workers so you have to change your red/green to maybe red/blue or provide multiple colour schemes.
Sometimes it is finding out what sort of problems customers face and to put the workrounds either in your documentation or on your website.
