That would be you but let’s say this is the scenario. You find a stain on the bathroom wall. Now you don’t believe in moisture meters so you ask for further evaluation by a professional.
The plumber says “I tore the wall open and there isn’t a plumbing leak. That will be $500 please.” The cost for repairing the drywall and tile $1000.
Customer hires a roofer. “I went into the attic and there was no leaks visible on the roof sheathing over the bathroom. However the HVAC air handler is above the bathroom. That will be $300 please”.
The HVAC technician says “Yes it looks like the air handler auxiliary pan leaked at one point, somebody installed a new one. There is no active leak here. That will be $300 please”.
So now you’re on the hook for about $2100. You have a pissed off customer. A moisture meter would have told you if that leak was active on the day of inspection. A good home inspector will go into the attic and investigate the source of the stain.