The level of confidence a team can get for the number of functional defects that a client can find in UAT has a direct relation to how much they know of the application they are building. You can write any number of scripts (manual or automated) but unless you get yourself to be in a position where you are like a user of the site you will never know if what you are testing will meet the client’s end requirements.
So at some point in time, get to know the application upfront even before you write a single line of code or test script or find someone who does. It’s no different from how our mind works when we travel from home to office or to a new place – either we know where we are going and our eyes and brain give is immediate and instant feedback if we go wrong or we need something like Google Maps to tell us (even though it is a bit delayed).
This is a step that you need to have to be able to deliver Quality on any project