Lazy me, inbstead of drawing a curve, I find a link to it

http://www.mitsubishichips.com/webfiles/...reliab.pdf

as you will see there is 'infant mortality' in semiconductors so a burn-in is a good idea.

HOWEVER:
This has at times showed up faults like insufficient heatsinks for regulators that shut down; unduly heating ICs / transformers due to some problem ; noise related issues when the unit crashes due to insufficient filter arrangements.
These are design problems and should NEVER have to be found during "burn in". This show reliance on so called "testing" which is pure BS.

succesful testing does not show the absence of bugs it shows the absence of known bugs

Erik