Suppose you pay a lot of money for a carbon bicycle, whether road or mountain bike. Your feeling tells you that a lot of money = lots of quality, right? Then you start riding but after a while your bike looks like this: Why a frames break, is explained here (link).

Or suppose you want to buy a bike through Ebay. Can you be sure that what on the surface looks like a gem is really flawless? Or they crash. Or worse, someone you loaned the bike to crashes but doesn’t tell you. How do you know the frame is still sound? The same applies after tightening the roof-rack clamp of derailleur clamp too tightly. On the surface, everything is OK but in reality the part is broken and only lasts a couple of days.
All of these questions can be answered through measurement. This website describes the measurement techniques, faces criticism and demonstrates some practical examples