Why we go and look
Drawings show what a system was designed to be. Site visits show what it actually is — how equipment has really been installed, what's changed since the last revision, where cabling was routed around an obstacle nobody documented. For electrical and electronic systems especially, the gap between "as designed" and "as built" is exactly where risk hides, and it rarely shows up if the only view of the system is on paper.
What it looks like in practice
That can mean walking a site with the client's own engineers to verify installation against design intent, attending factory or site acceptance testing, or simply being present when a system is commissioned so problems get caught by someone who understands the whole system, not just the piece in front of them. It's unglamorous work, and it's often where the most useful findings come from.