How often have you run into a weird behavior of the SuT and then tried hard to find evidence for this phenomena? Developers sometimes just don't believe you until you can prove with a video. Screen shots are no hard facts anymore, since they don't demonstrate what steps you have done aside.
The time that can be spent into investigation can grow dramatically. The more time you spend into finding the root cause, the more pressure on you to be successful so the time spent for searching is justifiable.
Woe betide you if you can't find it after so much time has been wasted.
Woe betide you if you cannot find the bug but the customer will.
Woe betide you if you found the bug, reported it, but someone still thinks it is reasonable enough for the customer...and then the customer doesn't like it...