Sorry, recently i have came across working with testers, previously i have tested website my self.
When testers do web User Interface Testing, they find silly ui bugs that users can't find and force devlopers (me) to solve those bugs.
They can't find important bugs which is related to backend programming etc.
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That just confirms that you need a multi-pronged test strategy involving users, UI testers, DBAs etc.
If your architecture is layered well then backend bugs should have been found during unit testing and subsystem testing. Maybe you are leaving too much testing until too late in the process?