Less popular in recent years but 5-10 years ago a lot of the cheaper printers like the ender 3 had problems with the build plate warping and it was easy and much cheaper to use a mirror or glass sheet since they are perfectly flat and worked just as well as what was available commercially at the time
Less popular in recent years but 5-10 years ago a lot of the cheaper printers like the ender 3 had problems with the build plate warping and it was easy and much cheaper to use a mirror or glass sheet since they are perfectly flat and worked just as well as what was available commercially at the time