Because the laser is essentially heat energy, if the lens or mirror is not transmitting the laser through it or reflecting it, it can build up heat. That heat can actually generate a crack. If you don’t clean your mirror or lens, you can actually crack them and cause permanent damage.
If you use a laser power meter, you could typically see about 5-8% reduction in output per mirror. If the mirror is dirty, and you have three mirrors, you could be as much as 15% off on your power output, which is significant.