Would you use an elevator that can handle up to 80kg ( before cable torns ) and you are 70kg?
I wouldn’t. What if something unexpected happens? What if you ate a bit more that day?
Would you live in a house that has walls that can hold exactly the weight you currently have in your house and not a single gram more? What if the wall gets just a little bit damaged? It wouldn’t be able to support the floor anymore and would collapse.
There is always a safety margin built into things all around you, everywhere.
Besides increasing general safety, it also increases reliability and lifespan of the product which saves money in the long term. And it is beneficial for the manufacturer as well - less RMAs, better reputation etc.