Preventative maintenance reduces the likelihood of plant breakdown and losses in manufacturing capacity while repairs are made. Adequate maintenance also helps to maintain the resale value of plant. Finally, ensuring the plant is producing within tolerances contributes towards the reduction of defects and improves your Internal Quality.
You should look at the Manufacturing Responsiveness Report and if you are losing a significant amount of time to Breakdowns, then consider increasing your spend on Preventative Maintenance.
Preventative Maintenance is one of your decisions under the Operations -> Process menu in the MikesBikes Advanced Business Simulation.