If you're developing an application, 
you'll want to make sure you're testing 
it under conditions that closely simulate 
a production environment. In production, 
you'll have an army of users banging away at your app
and filling your database with data, which puts stress on your code. 
If you're hand-entering data into a test environment
one record at a time using the UI, you're never going to
build up the volume and variety of data that your app 
will accumulate in a few days in production. Worse,
the data you enter will be biased towards 
your own usage patterns and won't match real-world usage,
leaving important bugs undiscovered.